Design puzzle : 2-input mux glitch issue

Problem statement: A 2-input multiplexer has both of its inputs getting value of "1". Will there be any toggle (glitch) happening at the output of the multiplexer? If yes, is that expected? What if both the inputs are getting value of "0"?

Answer: 

We all know that a multiplexer's output is equal to

IN0 if SEL = 0

IN1 if SEL =1

So, if both IN0 and IN1 are getting same logic value, output must not toggle. However, if we observe carefully, there is a high chance of a momentary glitch at the output in case both inputs are at value "1" and select toggles from "1" to "0". To understand this, we need to look into the internal structure of the mux, which is as shown in figure 2 below.

The figure says that output goes momentarily to "0" before finally settling down to "1". Why is this so? The reason behind this is the two paths going from SEL to OUT and toggling of both the inputs of final OR gate. And there is asymmetry of delays with one inverter being extra in one of the paths. This causes the output of the mux to go momentarily to zero.

Let us analyze this with the help of timing waveforms (assuming delay of each element to be 1 unit):

Thus, it is clear from the timing waveforms that there is a glitch in the output. It is possible to minimize the extent of this glitch by minimizing the difference of delays between the two paths getting formed between the SEL and OUT. However, it cannot be guaranteed even with greatest of precision during design as there are mismatches in fabrication of individual gates. So, even the best of multiplexers will have this limitation, however small it may be, unless designed specifically for this purpose. Can you suggest a design improvement that can help in this scenario?

One is forced to think here that what can be the consequences of such a glitch and what remedies can be there for this. I had written a post Glitches in combinational circuits that discussed what can be the consequences of glitches in combinational circuits. This scenario is a special case, but with some twist. Let us discuss all the cases one by one.

• If this case is in a data path for synchronous circuits, there is no issue as discussed in one of the points in our post Glitches in combinational circuits.
• If this case happens for a data path in asynchronous circuits, this can be an issue. So, synchronization circuits have to be designed with utmost care and following the rules of data synchronization
• If this scenario occurs in either path of clock or reset, this is an issue as this glitch can alter the state of the design by either letting the flop capture data at "D" pin by acting as an extra clock pulse, or can reset the flop.

Setup time and hold time - origin

In our previous post, Setup and hold – the state machine perspective, we discussed how setup and hold can be defined in respect of state machines. Interestingly, there is another perspective of setup and hold – that in repect to devices, known as setup and hold time requirements. For a device, (for example a flip-flop, a latch or an SoC), setup and hold times are defined as:

Setup time: Setup time of a device is defined as the minimum time before the clock edge the data should be kept stable so that it is reliably sampled by the clock.

Hold time: Hold time of a device is defined as the minimum time after the clock edge the data should be kept stable so that it is reliably sampled by the clock.

In other words, every device has a setup and hold window surrounding the active clock edge within which data should be kept stable. As is shown in figure 1, brown line represents the active clock edge, blue line represents setup window and red line represents hold window. As is shown, data can toggle at any time except between setup and hold windows. Toggling of data between setup-hold window means flip-flop might go into metastable state and the output of flip-flop does not remain predictable.

Figure 1: Setup and hold checks

Origin of setup and hold timing requirements: Let us consider a positive edge-triggered flip-flop. Figure 2 shows a most simplistic circuit for a practical flip-flop. Inverters I1, I2 and Transmission gates G1, G2 constitute master latch and I3, I4, G3, G4 constitute slave latch.

Figure 2: A typical practical circuit for negative edge-triggered flip-flop

Figure 3 below shows the origin of setup time requirement. For data to get latched properly, it should complete the feedback loop of master latch before the closing edge of clock at transmission gate G4. So, setup time requirement of the flip-flop is:

T_setup = T_G3 + T_I1 + T_I2 + T_G4

Figure 3: Figure demonstrating delays constituting setup check

Similarly, figure 4 below shows the origin of hold timing requirement. For data to get latched properly, the next data should not cross inverter I1. So, hold timing requirement of the flip-flop is:

T_hold = -(T_G3 + T_I1)

In other words, hold time is the minimum time required for the data to change after the clock edge has passed so that new data does not get captured at the present clock edge.

Figure 4: Delays constituted in hold check

Thus, in this post, we have discussed the origin of setup and hold checks for a device.