What is lockup latch in DFT?

A lock-up latch is a transparent latch used to avoid large clock skew and mitigate the problem in closing hold timing due to large uncommon clock path. From timing perspective lock-up latches can be the best solution to avoid large uncommon path between the clocks of two flops.

How do I install a lockup latch?

The hold check between lockup latch and domain2 flop is already relaxed as it is half cycle check. So, we can say that the correct way to insert a lockup latch is to insert it closer to launching flop and connect the launch domain clock to its clock pin.

What is clock mixing in DFT?

Majorly, in DFT, we avoid mixing different clocks in the same chain, but if there is a constraint to I/O ports we have to stitch scan flops driven by two different clocks in one chain.

What is time borrowing in latches?

Time borrowing is the property of a latch by virtue of which a path ending at a latch can borrow time from the next path in pipeline such that the overall time of the two paths remains the same. The time borrowed by the latch from next stage in pipeline is, then, subtracted from the next path’s time.

Why latches are made transparent during scan?

A positive level latch is transparent to the positive level(enable), and it latches the final input before it is changing its level(i.e. before enable goes to ‘0’ or before the clock goes to -ve level.)

What is setup time and hold time?

Setup time is defined as the minimum amount of time before the clock’s active edge that the data must be stable for it to be latched correctly. Hold time is defined as the minimum amount of time after the clock’s active edge during which data must be stable.

What is a scan flip-flop?

A scan flip-flop is a D flip-flop with a multiplexer added at the input with one input of the MUX acting as the functional input D, with the other input serving as the Scan-In (SI) input. Scan/Test Enable (SE/TE) is used to control the MUX selection bit. Scan flip-flops are used extensively for device testing.

What is shift mode in DFT?

Scan shift mode covers the shift paths when scan enable is high and scan capture covers the functional paths when scan enable is low. BOUNDARY SCAN. Boundary scan checks the stuck at faults at the boundary of the chip. ie, IO paths. Boundary scan flops are connected in a chain.

What are lockup latches?

A lock up latch is a sequential circuit which is used to address skew problems when multiple clock domains are used in a chip. From a DFT perspective it holds the previous scan data, and delays output transition so that the scan data can be effectively captured.

Does latch have setup and hold time?

yes a latch has setup and hold time in a way similar to f/f.

Why are inferred latches bad?

Inferred latches can serve as a ‘warning sign’ that the logic design might not be implemented as intended. A crucial if-else or case statement might be missing from the design. Latches can lead to timing issues and race conditions.

What is the difference between a latch and a flip-flop?

Both latches and flip-flops are circuit elements whose output depends not only on the current inputs, but also on previous inputs and outputs. The difference between a latch and a flip-flop is that a latch does not have a clock signal, whereas a flip-flop always does.

Is the lock up Latch and capturing flop the same?

Lock-up latch and capturing flop having the same clock (Will not solve the problem): In this case, the setup and hold checks will be as shown in figure 5. As is apparent from the waveforms, the hold check between domain1 flop and lockup latch is still the same as it was between domain 1 flop and domain 2 flop before.

When does a lock up Latch need to be inserted?

The polarity of the lockup latch needs to be such that it remains open during the inactive phase of the clock. Hence, Who inserts a lock-up latch: These days, tools exist that automatically add lockup latches where a scan chain is crossing domains.

What is the role of lock up latches in congestion?

Scan Lockup Latches – Significant Role in Congestion. Lock-up Latches play an important role in fixing timing problems especially for hold timing closure. A lock-up latch is a transparent latch used to avoid large clock skew and mitigate the problem in closing hold timing due to large uncommon clock path.

Why are lock up latches used in scan shift?

Lock-up latches are used in between the two scan flops having large hold failure probability due to uncommon clock path so that there is no issue in closing timing in a scan chain across domains in scan-shift mode. From timing perspective lock-up latches can be the best solution to avoid large uncommon path between the clocks of two flops.