C function that converts hexadecimal value to decimal value.

Hexadecimal to decimal conversion is something that is often needed in hardware. Below functions can be used for hexadecimal to decimal conversion in C:

#include<stdio.h>#include<conio.h>#include<string.h>
int get_value(char a)  { if(a>='0'&& a<='9' ) { return (a- '0'); } else if(a>='A' && a<='F') return ((a-'0')-7); } else if(a>='a' && a<='f') return ((a-'0')-39); else return -1;
}

int htoi(char a[]){ int len=strlen(a); int temp=0; for(int i=0;i<len;i++) { int digit=get_value(a[i]); if(digit == -1){ return -1; } temp=temp*16+digit; } return temp;}
int main(){ char a[]="f0"; clrscr(); int b=htoi(a); if(b == -1) printf("invalid input"); else printf("decimal value is %d",b); getch();        return 0;}

Interesting programming quiz : Array Bound Read Error

Problem: Can you figure out what is wrong with following piece of code?

#include <iostream>int main() {     int a[5] = {1,2,3,4,5};     for (int i = 4; a[i] >= 0 && i >=0 ; i--) {          std::cout<< "ith element of array is "<<a[i]<<std::endl;     }}

I would suggest you to  try it yourself before scrolling down to see the answer. Its quite interesting,

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Answer : Here, as one can figure out, the intention is to print array elements from end till we don't hit any negative number. In the first look it may seem fine but unfortunately it will end up in ABR (Array Bound Read).

Explanation : After the completion of 5th iteration; i.e. when i = 0, compiler will decrement i;  i.e., i will become "-1". It will, then, try to check the condition, which will result in reading a[-1]. Since, array can have indexes only greater than or equal to 0, it will result in an error. Trying to read array elements out of the allowed indexes is termed as Array Bound Read Error. Hence, one should avoid such conditions because it can result into random result. The program can crash anytime. If you are lucky, it may run successfully also. Its all up to your luck.  Instead, it should be

for (int i = 5; i>=0 && a[i]  >= 0 ; i--) {

i.e. first check index value and then do the array access operation.

Here, with the above solution, one more interesting thing comes up to understand. In AND (&&) operation compiler first evaluates  condition1; if it is true, then goes to evaluate condition2; otherwise return false from there only.

For example,

#include <iostream>
int main() {int i = 0;int j= 1;if(  ( i == 1) && (++j ==3)  ) {      std::cout<<"inside if"<<std::endl;}std::cout<<"i is "<<i<<" and j is "<<j<<std::endl;}

Output :

i is 0 and j is 1

Here, as you can see code control will not go into if branch as none of condition is true. since condition1 i==1 is false, compiler will not even check condition2 i.e. value of j will not be incremented.

Internally,  compiler might be doing some following kind of transformation to evaluate && operation

  bool cond = (i==1);  if( cond ) {      cond = (++j != 0) ;  }if(cond){      std::cout<<"inside if"<<std::endl;}

Also read:

• Automatic include of header dependencies in makefile
• Library
• C++ trivia
• Some good reads about process
• String class vs dynamically allocated array

Function Overloading

Function overloading is a feature inherent in many programming languages including c++. It allows a user to write multiple functions with same name but with different signatures. On calling the function, the version of the function corresponding to the signature will be referred to. Function signature includes function parameters/arguments, but it does not include return type. Function signature may differ in terms of number of parameters or type of parameters. Let us illustrate with the help of a few examples:

Example 1: The two functions below are overloaded, since the return type of arguments differ:

int func(int a,int b);double func(double a,double b);

Example 2: The two functions below are overloaded because they differ in the number of arguments:

int func(int a,int b);int func(int a,int b,int c);

Example 3: The two functions below are not overloaded because they differ only in terms of their return type; the number and type of all the arguments is same.

void func(int a,int b,int c);int func(int a,int b,int c);

Please note that C does not support function overloading because there is no concept of name mangling in C. On the other hand, C++ does support function overloading as name mangling is supported in C++. Name mangling is mangled name of function name and its signature which is used by C++ compiler internally to refer to functions. For instance, in above example 1, mangled name of functions will look something like shown below:

func__int_intfunc__double_double

This way C++ compiler can handle function overloading. 

Note : above are not the actual mangled names. Compiler can make some more complicated names. this is just for understanding. 

Also read:

• Meyer's singleton pattern
• Function overloading
• Comparison between array, linked list and vector
• C function for hexadecimal to decimal conversion
• Problem - can you figure out is there something wrong with this code

On-chip variations – the STA takeaway

Static timing analysis of a design is performed to estimate its working frequency after the design has been fabricated. Nominal delays of the logic gates as per characterization are calculated and some pessimism is applied above that to see if there will be any setup and/or hold violation at the target frequency. However, all the transistors manufactured are not alike. Also, not all the transistors receive the same voltage and are at same temperature.  The characterized delay is just the delay of which there is maximum probability. The delay variation of a typical sample of transistors on silicon follows the curve as shown in figure 1. As is shown, most of the transistors have nominal characteristics. Typically, timing signoff is carried out with some margin. By doing this, the designer is trying to ensure that more number of transistors are covered. There is direct relationship between the margin and yield. Greater the margin taken, larger is the yield. However, after a certain point, there is not much increase in yield by increasing margins. In that case, it adds more cost to the designer than it saves by increase in yield. Therefore, margins should be applied so as to give maximum profits.

Number of transistors v/s delay for a typical silicon transistors sample

We have discussed above how variations in characteristics of transistors are taken care of in STA. These variations in transistors’ characteristics as fabricated on silicon are known as OCV (On-Chip Variations). The reason for OCV, as discussed above also, is that all transistors on-chip are not alike in geometry, in their surroundings, and position with respect to power supply. The variations are mainly caused by three factors:

• Process variations: The process of fabrication includes diffusion, drawing out of metal wires, gate drawing etc. The diffusion density is not uniform throughout wafer. Also, the width of metal wire is not constant. Let us say, the width is 1um +- 20 nm. So, the metal delays are bound to be within a range rather than a single value. Similarly, diffusion regions for all transistors will not have exactly same diffusion concentrations. So, all transistors are expected to have somewhat different characteristics.
• Voltage variation: Power is distributed to all transistors on the chip with the help of a power grid. The power grid has its own resistance and capacitance. So, there is voltage drop along the power grid. Those transistors situated close to power source (or those having lesser resistive paths from power source) receive larger voltage as compared to other transistors. That is why, there is variation seen across transistors for delay.
• Temperature variation: Similarly, all the transistors on the same chip cannot have same temperature. So, there are variations in characteristics due to variation in temperatures across the chip.

How to take care of OCV: To tackle OCV, the STA for the design is closed with some margins. There are various margining methodologies available. One of these is applying a flat margin over whole design. However, this is over pessimistic since some cells may be more prone to variations than others. Another approach is applying cell based margins based on silicon data as what cells are more prone to variations. There also exist methodologies based on different theories e.g. location based margins and statistically calculated margins. As advances are happening in STA, more accurate and faster discoveries are coming into existence.

Also read:

• Power aware RTL design
• Lockup latch - principle, application and timing
• Propagation delay of logic gates
• Defining a clock signal in VHDL