MY GATE 2015

Many people who are reading this blog might have asked themselves what is this person doing now??

So, having being frustrated with the work, I along with few other friends have started preparing for GATE. Four of us prepared and all four of us being from Infosys managed to prepare well and end up getting good ranks.

Shivananda Rapolu – AIR 303 ( AIR 400 odd in Gate 2014 ) - Seat in IISC DESE now in 2015.

Sreenu Kakunuri – AIR 141 ( AIR 500 odd in Gate 2014) - Seat in Kgp VIPES now in 2015.

Mahendra Mundru – AIR 22 ( CSE )  ( AIR 300 odd in Gate 2014 ) - Seat in IIT Bombay in 2015.

Raghavapudi VNK Neeraj ( Myself ) - AIR 70 ( AIR 751 in Gate 2014)- Seat in IIT Delhi VDTT in 2015.

You can claim that all of us have improved our ranks but then we had to write Gate again because we didn't knew many things like the Mtech RA of IIT bombay which is super for our ranks last year ones,

IIT Madras TI MS and regular MS in VLSI which are just awesome, IIT Delhi Computer Technology, JTM, JCA, Hyderabad RA and Direct Phd programmes in all IITs

Don't take rest post gate. Travel a lot. Appear for a lot of Interviews... Run Run. Run.. 

But we all had to take the pain of writing GATE again while working .

My biggest gratitude to My Parents, who have been always with me and supporting me, My Philosopher, my guide - Mr. Sudhakar Reddy ( My college HOD ), All my friends and in specific  my buddy - Harish Kumar ( studying Mtech Communications in IIT hyd).

Without Harish, a lot would have not been possible. Thanks to him from the bottom of my heart.

By now, you would have got the reason why I blogged this information. I don’t want any one to miss things because he has no information regarding it.

Hope this helps....

Getting AIR 70 didn't stop my struggles..  I travelled to many IITs still........... 

Keep viewing and get the rest of the story - Of how I failed in IISC CEDT and how I was out smarted by the Texas and How did I outsmart the Delhi Professors.. 

Keep Viewing. . 

                                  

The Unknown Mtech degrees ?

1. IIT Madras Regular and Texas M.S are very good and often people over look it. Don’t do it.

 2. IIT Delhi VDTT

 3. IIT Delhi JTM   and Computer Technology  

                                                                   

M.S. in India........

          Its been few years that research is getting good boost up in India. M.S. is one of the prominent courses to have got started in IITs. Not all IITs over it. IIT madras, delhi, kgp offer M.S. All the three are good.

Though the degree is equal academically with Mtech, the duration will be round 1.5 to 2.5 yrs. It is more of research oriented and less of courses.  It is like MINI Phd. as one of my friend Karishma (M.S. @ IIT kgp) says

Don't compare Mtech and MS that IITs offer. Both are different courses meant for different purposes.

Admission is mainly based on written test and Interview. But GATE is mandatory

Shortly I will add a write up regarding MS in IITs and about research. It will be from my friend Karishma, pursuing her MS in IIT Kgp.  

Why not Direct PHD...

Yes, many don't know about this. There is provision of direct PHD in India and few IITs have integrated course like the IIT Bombay which has Mtech+PHD course. Again Admission is mainly based on written test and Interview. But GATE is mandatory.

Understand Yourself First before deciding whether to take research or conventional courses

M.S. vrs Mtech

Both are good. MS is research oriented. If your research is in commercially viable areas, surely you will get placed in Companies in R&D sector. If not, you can go for Phd and plan for an academic career.

                  

 Mtech  RA good or bad

Institute RA

 1. You will work in any one of the laboratories (System Admin lab, communications and DSP lab, microelectronics lab, power electronics and machines lab, electronic circuits lab) of the institute. Your work may be heavy or light depending on the lab you get.
2. You are free to choose your final year project and guide based on your interests.
3.For shortlisting for interview, 50% weightage is given to GATE score and 50% weightage to your marks in the written test

Project RA

1.You will be assigned  a particular project under a particular professor. This may be an institute sponsored project or a project sponsored
2.Gate Score weightage is 75%
3.You are not free to choose your final year project and guide

More information about IIT Bombay RA is given in the later posts where I have shared one of my friends' experience and I bet it is totally worth it reading.

As I said that there is no difference in the degree you get. All are same and it depends on you  as to how well you do.

With this, I will end up telling you things and now start telling you things that made me get good rank in GATE 2015.
 What all did I study? 
        How did I manage time inspite of working in a company?
                   What books did I study?
        
                                              Come on explore more for all the secrets.

The Others

IIT Roorkee and Guwahati

Both have only Mtech courses (TA).  They conduct counseling for their seats and few times interviews. They have VLSI, Communications, Signal Processing branches.

Check out with the seniors before you join them.

As it is counselling based. it is tought to estimate. 

AIR< 300 ( General Category )   - VLSI

AIR< 450 ( General Category )  - Communication and Signal Processing.

Newer IITs

Of all the Newer IITs, only IIT Hyderabad is promising and is very super in terms of standards. This I am saying because of lot of reasons. I had done my Internship in IIT Hyderabad. It is very good in terms of research and placements and facilities, inspite of being a new IIT.. 

I have lot of friends who are studying and none of them have raised any negative point regarding it and all of them are getting into good companies. 

IIT Hyderabad

It offers MTECH TA and RA, Direct Phd. Since 2015, It started Counseling for TA seats. Until 2014, it used to have direct admission for TA and Written test for RA.

1. MTECH - TA

AIR< 250 ( General Category ) – VLSI

AIR < 500 ( General Category )  - Communication and Signal Processing

2 MTECH – RA

AIR < 1000 ( General Category ) will get call for written test and Interview

3 Direct Phd

AIR < 4000 (General Category )  - Call for Written test and Interview

Upto now, I have discussed maximum number of specializations of Post Graduation in IITs for ECE guys.

With this I will summarize this discussion and discuss a bit about other things.

IIT Kharagpur

The oldest IIT and often called as the best for ECE. It has direct admission for Mtech

6.1 Mtech – TA

        AIR < 100 ( General Category )  - VLSI

        AIR < 300  ( General Category )  - Communication Signal Processing

        AIR < 250 ( General Category )  - VIPES – Video and Image Processing

        AIR < 400 ( General Category ) – RF and Micro Wave

        AIR < 500 ( General Category ) – Multimedia and Image Processing

There are few Mtech – TA  specializations which have written test/ Interview. Kindly get noted regarded it.

6.2 MS and Direct Phd

Same Arguments applicable as any other IIT

If you have any specific doubts regarding things, you can comment here and I will try to answer to the best of my knowledge regarding rank and other doubts.

Let us see the remaining IITs. I will be putting information regarding only only top IITs. I haven't included IIT bhuvaneswar, indore, gandhinagar and so on...

IIT Delhi

Often the second best place after IIT Bombay in terms of placements. It has lot of inter disciplinary courses and often ECE people find interest in here.  

Again IIT Delhi also offers Mtech ( TA Only no RA ), MS By research , Direct Phd.  The arguments and information about the latter two are same.

IIT Delhi conducts written test/ Interview for all its branches. No direct admission. Often it has high weightage to Gate score.

5.1  Mtech TA - Interviews

AIR< 80 ( General Category ) - IEC – Integrated Electronics and Ckts   Call for Interview

AIR < 150 ( General Category ) – Communications and Signal Processing

AIR < 900 ( General Category ) – Computer Technology ( No Gate score weightage here)

5.1.1 Interdisciplinary Mtech Courses for ECE Guys :

AIR < 300 ( General Category )  - JTM : Telecommunications and Management.

AIR < 200 ( General Category )  - JVL – VDTT ( VLSI Design Tools and Technology)

JTM : It is communications branch which is very good in terms of placements in software industry.

VDTT : It is a sponsored programme where in the first round of interviews are conducted by the IIT Faculty and the second round by the companies who have come up to sponsor you.

It is best programme in India in terms of knowledge because it has industrial tie up and you will be getting a pre placement offer and also an opportunity to intern after your first year in their company. It has no Reservation. 

Computer Technology – Very good for those who want a software placement.

 5.2 MS by Research

 In all allowed specializations

5.3. Direct Phd

In all allowed specializations

The advantage with IIT Delhi is that apart from the academics, it is the best place to be in for IAS and other Civils Coaching. 


If you have any specific doubts regarding things, you can comment here and I will try to answer to the best of my knowledge regarding rank and other doubts.

Next IIT is the one that was established the first. 

IIT Kanpur

The best place to be in after IISC and IIT Madras for research is un doubtedly IIT Kanpur. It is very good.  They have only Mtech and recently MS has been started. But I am putting my information only regarding Mtech.

IIT Kanpur has only 2 yr Mtech in VLSI ,Communications and RF. ( Apt and Useful to us)
From 2015, they also started MS, but I have very little information, I will update it soon since one of my friend ended up joining it and got placed recently in Dec 2016 in Intel !!!

They conduct interview/written test.  The number of seats are high on par with other IITs in each specialization. But the common myths are that the lower rankers like < 150 are not entertained there mostly because the management doesn’t want any one to leave their college  before completing their MTech for government job and waste a seat. So they ensure no one is there of that category. But this is common myth and need not be true.

AIR< 300 ( General Category )  - VLSI Call for Interview

AIR< 500 ( General Category )  - Communication Call for Interview

AIR < 700 ( General Category )  - RF Call for Interview

Since 2015, IIT Kanpur is starting MS also.  Check out once. 

If you have any specific doubts regarding things, you can comment here and I will try to answer to the best of my knowledge regarding rank and other doubts.

Next IIT is our Capital City College. . I know. all know. it. 

IIT Madras

It is located in the wooden campus. It looks like a forest. One of the best places to be in for a quality research in VLSI.

Remember that IIT Madras will be too late in announcing the results so as a result the analysis regarding it is very tough. But there is also less seats here on par with other IITs and also moreover they have spot admission. With this coming into picture, it makes a bit difficult to guess about the whereabouts of the analysis.

Still generally We can note the following things in general. Also there is no 3 yr Mtech – RA here. There is a MS by Research Programme. ( It is like MSc in IISC )

3.1 Mtech – 2 yr – TA – Direct Admission

           AIR<50 ( General Category )  - VLSI  ( If not in the first round, atleast in the later rounds)

          AIR < 100 ( General Category  ) – Communications.

3.2 MS By research. – Written and or Interview

1.       NORMAL / REGULAR MS :  It has written test and approximately upto 2000 ranks in the open category are called for the written test and Interview process. No weightage to Gate score. It is not a time bound course and professor bound course. But MS in VLSI is highly required now a days.

2.       TEXAS MS :  It is very good programme with the sponsorship coming from the Texas Company and an offer of employment with them post your MS. You will be paid minimum of 20K per month as stipend and also an opportunity with Texas as an Intern. The written test and Interview has strictly < 1000 rank in any category. There is no reservation.

For more information : http://www.ee.iitm.ac.in/vlsi/research/start

3.3    Direct Phd / Phd – Written Test and or Interview

This is again a research programme and Direct Phd is taken immediately after Btech and normal Phd is after your Masters.

If you have any specific doubts regarding things, you can comment here and I will try to answer to the best of my knowledge regarding rank and other doubts.

Next IIT is none other than the IIT known for the most packages in 2014.. Guesses come one. 

IIT Bombay

Often called as home for placements, it is located in Powai. The best IIT for placements. It is very good in terms of the infrastructure and Industry tie ups.

1.1   Mtech – TA – Teaching Assistantship – 2 yr Program – Direct Admission

Specializations are 6 But I am telling only those that will land you in job or rather in one or other way related to ECE.

Micro Electronics, Electronic Systems, Communications Engineering.

1.2   Mtech – RA – Research Assistanship – 3 yr Program – Written Test and Interview

It has same specializations as what Mtech TA has. The difference is that it is relaxed program because it has 1 extra year. But there are two types of RA – Project RA and Institute RA.

The job opportunities are same  for both RA and TA except that the RA guys spend 1 extra year in IIT Bombay.  But be prepared for some higher amount of work hours than the TA guys.

1.3   Mtech + Phd – 5 yrs Minimum – Written Test and Interview

It has same specializations.

Getting an admission into TA is very tough because it has direct admission. RA and Phd program is much more relaxed. So higher rank guys can try for this.

AIR< 40 ( General Category )  -  TA Micro Electronics

AIR< 70 ( General Category )   - TA Electronic Systems

AIR< 90 ( General Category )   - TA Communication Systems

AIR < 700 ( General Category ) – Mtech + Phd Call Letter

AIR <  1000 ( General Category )  - RA Written Test Interview. The number of seats in RA category are not fixed and ad hoc. They are as per the needs of the Professors.

I have put up some information regarding the types of RA in the later parts. 

If you have any specific doubts regarding things, you can comment here and I will try to answer to the best of my knowledge regarding rank and other doubts.

                   Next IIT is Wooden Palace. . Man.. Check it out. 

IISC Bangalore

IISC Bangalore

IISC closes its applications very fastly mostly 1 week after Gate Results. So Be aware.  Other IITs give you some time.  Be be prepared with all the necessary things like scanned copies of your Btech Marklists , Original Degree,  Photo, Signature. If you are that year pass out, then have Course Completion certificate. Remember that most of the Institutes will ask 60% Btech Percentage Minimum.

Located in Bangalore and often called as the best place to be in for a peaceful study. If someone is interested in Research Programmes. This is a heaven. I am here by telling the programmes it offers.

1.1  ME : Master of Engineering  - Direct Admission

          Specialization :  Micro Electronics, Signal Processing,  Communications and Networks.

1.2. Mtech : Master of Technology 

  CEDT ( Centre of Electronics Design Technology). Now it is called as DESE ( Dept of Electronic System Engineering). Written Test and Interview.

1.3  MSc Engineering  - Written Test and or Interview

This is a research program and the duration will not be fixed and generally it takes 2 to 3 years. It can be opted by those who are interested in research.  But just remember that when you are in some prestigious institute, then you should not think much about placements and think only  about gaining as much knowledge as possible.

In placements they open for a Department and as long as you are in the same Electrical Department, you should not be much worried whether you are ME/MTECH/MSc. It is your knowledge levels that matter. But in MSc you should be prepared to do more research work rather than conventional course work unlike Mtech/M.E.

This is also one of the options. 

1.4 Direct Phd / Phd – Written Test and or Interview

This is again a research programme and Direct Phd is taken immediately after Btech and normal Phd is after your Masters.

So getting an admission for ME Programmes is very tough because it is direct admission. So Gate ranks should be very low. But CEDT is a VLSI Programme and it is one of the best in India.  It has a written test followed by Interview. But the advantage here is that there is no weightage to Gate score or your previous stage performance in the evaluation of your next stage.

AIR< 20 ( General Category)  - ME  Micro Electronics.

AIR< 120 ( General Category)   - ME  Communications and Networks and ME Signal Processing.

AIR < 300 ( General Category )  - CEDT call letter you will get. Prepare well from Gate results. Contact those who are studying their First Year CEDT and get information about CEDT.

AIR>300 & AIR<1000 ( General Category )  - MSc Program and Direct Phd  call letter is for sure. So be prepared for it if you want to go for such programmes. 

Remember for OBC you can assume 1.5 times and SC/ST 3 times atleast. But when you have category, better apply because it depends on the number of applicants ahead of you. The seats are reserved and if there is no one in your category then inspite of how large your rank is you will be getting it. So don't miss what you have. 

If you have any specific doubts regarding things, you can comment here and I will try to answer to the best of my knowledge regarding rank and other doubts.

This is all about IISC programmes, that I wanted to put up. 

                   Let us see the Bombay IIT in the next IIT.........

IIT Mtech or MS in Foreign

Al though the above mentioned reasons are not the only but few among my mind.. Once you are stubborn that you want to pursue higher education, the next idea will be M.S. in Foreign Universities or M Tech in IITs?  ( I have not put any ideas regarding the Non Technical Education)

Why M.S in Foreign Universities.  :  

If you have good family background and is someone who can fight out in odd environments, then you can try out this. But be stubborn that the amount is going to be high but then you can do some part time and other things to get back some time. The placements are good in many universities and you can end up easily with 40 to 50 Lakh Salary Jobs.

In Foreign PHD are paid scholarships easily on par with M.S. But for M.S. it is tough to get scholarship. Also develop some clarity as to why you are going for M.S. Is it because of your interest in Higher education and research or is it for better placement. Depending upon that you have to choose M.S. with Thesis or without thesis. Know about these things better.

Why M Tech in India :  

Because if you think your family is not that good financially and you want to do it free of cost, then this is the best place. Join M tech and you will get some financial aid of 12400 Rs if Mtech and if Phd it will be 28000 Rs per month. So it is Risk free and moreover if you feel that you develop even more interest towards studies after doing your masters, then you can surely continue your education in same IIT as PHD scholar or also go to foreign university for your PHD. But the advantage will be that you will be paid scholarship for your PHD in Foreign Universities.. Or if you felt you have done enough by studying Mtech in IITs you can easily get placed in IITs or you can opt for lecturer jobs in any college even in the worst case.

If you decide upon IIT Mtech then you can carry on reading the blog. Because I have put the information only regarding it only.  Just to remind you that the information given below regarding Gate ranks is mostly about GATE ECE only. Although the above discussion that We had is applicable to any branch.

For those who feel they can take it up - go for MS or else Mtech in IITs is always a good and a stepping stone for a better career.

So at this point of time, I know many would have thought about their careers. So few of your friends might have already started their dreams of enjoying in Foreign and few others would have started their dreams about IITs.
                     
Let me put up the information about MTech in IITs - ( India First after all)
                                 

Why higher education

 1. If you are one of the techie guys and is passionate about doing some silly and funny  things with technology – Go for Mtech in IITs

2. Most of us study only in small colleges not known much for placements. So always a higher degree in top colleges will help you get better placed and better brand.

3. Passion towards Education may be the reason for few. They needn't stop their education after Btech and start working in some companies too early.

4.In B Tech, most of students end up getting placed in companies like Infy, TCS, Wipro, CapGemini and few others which are bulk recruiters. I do agree that at that point, one would always get carried away and hence will not think about any other thing apart from the campus placements. 

Remember that most of these companies only believe your analytic power and offer you employment. The package they pay you will be pretty low like 3 to 4 Lakh at max depending on which company you land up. You will be trained on something and finally you will be working on something which may or may not like it or know it rather. If you hate all these, then better go for higher education...............

5. Want to work for core Companies........... of ECE. then go for higher educaiton...

6. Often Post graduation is the stepping stone for one's career in research and high profile jobs.

7. Last but never the least .....Higher package….  in IITs.. Surely average package is 10 Lakhs

 ( Reminding you again that all this is for ECE guys. Because I know most of my CSE friends of Mtech will be earning  20s and 30s Lakhs on an average)

So, having seen these points, I know there is a bit of confusion among the minds of many a students.  
                Few would have thought of MS in foreign and few would have thought of Mtech in IITs.

What to choose?? 
                                           Why Mtech ? Why MS..
                                          What are types of Mtech and MS

                                          Is MS equivalent to Mtech in India?
                                          Is MS the property of USA or is it present in India also??

Thinking right?? 
                                                          Carry on guys. there is lot more in the next post.

How propagation of ‘X’ happens through different logic gates

‘X’ refers to a signal attaining a value that is ‘unknown’. It can be either ‘0’ or ‘1’. But, the exact value of the signal is not known. If a simulator is not able to decide whether a logic value should be logic ‘0’ or logic ‘1’, it will assign a value ‘X’ to the value. An example of ‘X’ source is a logic block that has not been initialized properly through reset. Having an ‘X’ value at a node can propagate to the logic lying in the fan-out, thereby increasing the uncertainty downstream the logic.

How ‘X’ propagates: An ‘X’ value at the input of a logic gate may or may not propagate to its output depending upon the states at other inputs of the logic gate. Given below is how different logic gates react to ‘X’ values:

1)  OR gate: An OR gate can absorb an ‘X’ if the other input has logic ‘1’. Otherwise, ‘X’ propagates through it. Please refer figure 1 for explanation:

Figure 1: X-propagation through OR gate

2) AND gate: An AND gate can absorb ‘X’ if the other input has logic ‘0’. Otherwise, ‘X’ propagates through it. Please refer figure 2 for explanation:

Figure 2: X-propagation through AND gate

3) Buffer/inverter: Since, buffers/inverters are single input gates, an ‘X’ at the input means ‘X’ at the output.

4) XOR gate: An ‘X’ at one of the inputs of XOR produces ‘X’ at the output no matter what the 

other input state is. Please refer truth table given in Figure 3 for explanation:

Figure 3: X-propagation through XOR gate

5) Complex gates: For complex gates, whether ‘X’ will propagate to the output depends upon the overall function of the ‘X’ input with respect to other gates. E.g. suppose a gate with function

Z = A + (B * C)

Then, if B input goes ‘X’, the output will go ‘X’ if A=0 and C=1.

Read also:

• Carry Look Ahead Adder
• MBIST (Memory Built-In Self Test)
• Controllability and observability - two DFT principles
• Digital counters
• Regions of operation of MOS transistors

Can a net have negative propagation delay?

As we discussed in ‘’Is it possible for a logic gate to have negative propagation delay”, a logic cell can have negative propagation delay. However, the only condition we mentioned was that the transition at the output pin should be improved drastically so that 50% level at output is reached before 50% level of input waveform.

In other words, the only condition for negative delay is to have improvement in slew. As we know, a net has only passive parasitic in the form of parasitic resistances and capacitances. Passive elements can only degrade the transition as they cannot provide energy (assuming no crosstalk); rather can only dissipate it. In other words, it is not possible for a net to have negative propagation delay.

However, we can have negative delay for a net, if there is crosstalk, as crosstalk can improve the transition on a net. In other words, in the presence of crosstalk, we can have 50% level at output reached before 50% level at input; hence, negative propagation delay of a net.

Also read:

• Bubble errors and bubble error correction
• Noise margins
• Can hold be frequency dependant
• Worst slew propagation
• Latchup in CMOS devices

Temperature inversion – concept and phenomenon

To understand the phenomenon of temperature inversion, let us first understand the concepts governing the conductivity of semiconductor devices with respect to changes in temperature.

Phenomenon governing semoconductor conductivity vs. temperature: In all, there are two phenomenon that govern the conductivity in any device-

• Carrier concentration: Electrons and holes are the charge carriers in a semiconductor. More is the number of carriers; greater is the conductivity of the material. Rise in temperature causes greater number of bonds to break due to higher number of collisions among vibrating molecules; thus, resulting in higher number of carriers with increase in temperature. This factor tends to increase the conductivity with increasing temperature. More the number of carriers, greater is the conductivity.
• Mobility of the carriers: Mobility is another measure of conductivity. Greater is the mobility of carriers, carriers move with greater speed, thus, contributing more to the overall current; hence, greater is the conductivity of the material. With increase in temperature, lattice vibrations increase resulting in less mobility of free carriers. So, this factor tends to decrease the conductivity with temperature increase.

Summing up, the trend of conductivity with temperature depends upon which of the above two factors dominates. Based upon the conductivity, the materials can be divided into three types - conductors, insulators and semi-conductors. Let us explore how the conductivity of these materials is based on the above two factors:

Conductivity of conductors (metals): Metals have abundance of loosely attached nearly free electrons (as is commonly called, the electron sea), the carriers of electric current. The increase in carrier concentration is ignorable with change in temperature. So, mobility factor dominates. The conductivity of conductors decreases with increase in temperature.

Conductivity of insulators (non-metals): Insulators have almost negligible free carriers. The electrons in insulators are tightly bound to atoms by bonds. The conductivity is negligible in insulators due to limited number of carriers. However, the number of free carriers increases exponentially with temperature. This increase in carrier concentration with temperature outpaces the decrease in mobility thereby making the insulators to gain conductivity with rise in temperature. So, the conductivity in insulators increases with rise in temperature.

Conductivity trend in Semiconductors: Semiconductors have conductivity in-between metals and insulators. These are the class of insulating materials in which electrons are loosely bound to atoms. A small energy is needed to break these bonds and supply free carriers, which can be supplied by potential difference applied across the semiconductor, or, by temperature itself, in the form of thermal energy. So, there can be any of the two factors dominating depending upon the voltage applied across the semiconductor. The decrease or increase in conductivity of semiconductor depends upon which of the two factors dominates. For CMOS transistors, the number of charge carriers directly translates to threshold voltage.

At high voltage levels applied, there is abundance of free charge carriers as a result of the energy supplied by the potential difference created. At this state, there is not significant change in carrier concentration with increase in temperature; so, the mobility factor dominates; thereby, decreasing the conductivity with temperature. In other words, at high levels of voltages applied, the conductivity of semiconductors decreases with temperature.

Similarly, in the absence of any voltage applied, or with little voltage applied, the semiconductor behaves similar to an insulator with very less number of carriers, those resulting from only thermal energy. So, increase in carrier concentration is the dominating factor. So, we can say that at low applied voltages, the conductivity of semiconductors increases with temperature.

The concept of temperature inversion: With reference to the discussion we had earlier, at higher technology nodes, the voltage levels used to be high. So, traditionally, the delay of CMOS logic circuits used to increase with temperature. So, the most timing critical corner used to be worst process, minimum voltage and maximum temperature. However, with scaling down of technology, the voltage levels have also scaled down. Due to this, at sub-nanometer technology levels, both the factors come into play. At lower range of the operating voltage levels, first factor comes into play. In other words, at lower technology nodes, the most setup timing critical corner has become worst process, minimum voltage and minimum temperature. This shift in setup critical corner, in VLSI jargon, is termed as temperature inversion.

Also read:

• Interesting design quiz : BCD multiply by 5 circuit
• Latchup in CMOS design
• Our world - digital or analog
• Power aware RTL design
• Race conditions

Can hold check be frequency dependant?

We often encounter people argue that hold check is frequency independent. However, it is only partially true. This condition is true only for zero-cycle hold checks. By zero cycle hold checks, we mean that the hold check is performed on the same edge at which it is launched. This is true in case of timing paths between same polarity registers; e.g. between positive edge-triggered flops. Figure 1 below shows timing checks for a data-path launched from a positive edge-triggered flip-flop and captured at a positive edge-triggered flip-flop. The hold timing, in this case, is checked at the same edge at which data is launched. Changing the clock frequency will not cause hold check to change.

Figure 1: Setup and hold checks for positive edge-triggered to positive edge-triggered flip-flop

Most of the cases in today’s designs are of this type only. The exceptions to zero cycle hold check are not too many. There are hold checks for previous edge also. However, these are very relaxed as compared to zero cycle hold check. Hence, are not mentioned. Also, hold checks on next edge are impossible to be met considering cross-corner delay variations. So, seldom do we hear that hold check is frequency dependant. Let us talk of different scenarios of frequency dependant hold checks:

1.  From positive edge-triggered flip-flop to negative edge-triggered flip-flop and vice-versa: Figure 2 below shows the setup and hold checks for a timing path from positive edge-triggered flip-flop to a negative edge-triggered flip-flop. Change in frequency will change the distance between the two adjacent edges; hence, hold check will change. The equation for hold timing will be given for below case as:

T_data + T_clk/2 > T_skew + T_hold

or

Tslack =  T_clk/2 - T_hold - T_skew + T_data

          Thus, clock period comes into picture in calculation of hold timing slack.

Figure 2: Setup and hold checks for timing path from positive edge-triggered flip-flop to negative edge-triggered flip-flop

Similarly, for timing paths launching from negative edge-triggered flip-flop and being captured at positive edge-triggered flip-flop, clock period comes into picture. However, this check is very relaxed most of the times. It is evident from above equation that for hold slack to be negative, the skew between launch and capture clocks should be greater than half clock cycle which is very rare scenario to occur. Even at 2 GHz frequency (T_clk = 500 ps), skew has to be greater than 250 ps which is still very rare.

Coming to latches, hold check from a positive level-sensitive latch to negative edge-triggered flip-flop is half cycle. Similarly, hold check from a negative level-sensitive latch to positive edge-triggered flip-flop is half cycle. Hence, hold check in both of these cases is frequency dependant.

2. Clock gating hold checks: When data launched from a negative edge-triggered flip-flop gates a clock on an OR gate, hold is checked on next positive edge to the edge at which data is launched as shown in figure 3, which is frequency dependant.

Figure 3: Clock gating hold check between data launched from a negative edge-triggered flip-flop and and clock at an OR gate

           Similarly, data launched from positive edge-triggered and gating clock on an AND gate form half cycle hold. However, this kind of check is not possible to meet under normal scenarios considering cross-corner variations.

3)      Non-default hold checks: Sometimes, due to architectural requirements (e.g. multi-cycle paths for hold), hold check is non-zero cycle even for positive edge-triggered to positive edge-triggered paths as shown in figure 4 below.

Figure 4: Non-default hold check with multi-cycle path of 1 cycle specified

Also read:

• Lockup latch - principle, application and timing
• Data checks - all about data setup and data hold checks
• Worst slew propagation
• On-chip variations - the STA takeaway