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AR# 3006

CPLD XC9500/XL/XV - How do I calculate the timing across a latch?

Description

General Description:

The timing report (design.tim) produced by the CPLD software does not show timing for any paths across a latch. What is the correct way of calculating timing across a latch?

Solution

Transparent latches are implemented in 9K devices by gating the D and G inputs into the async CLR and PRE inputs of a D-flip-flop (FDCP primitive). Hence, the delay calculations would be the same

as when tracing a path through the async clear or preset of a D-flop.

The logic equations for a 9K latch are:

Q.preset = D * G

Q.clear = /D * G

The pin-to-pin propagation delay for both the data and latch-enable paths in a 9K device would be calculated as follows, if the D and G inputs are driven by IBUFs and Q drives an OBUF.

tPROP = tIN + tPTSR + tAOI + tOUT.

The values for tIN, tPTSR, tAOI and tOUT can be found in the data book and are dependent on the speed grade being used.

If there is any additional logic on the D or G inputs, the logic would usually require an additional macrocell delay (tF + tLOGI + tPDI). The only logic that could possibly be implemented in the same macrocell as the latch (requiring no additional delay) would be a single AND-gate (any size) on the G path (but not on the D path).

AR# 3006
Date Created 08/21/2007
Last Updated 12/15/2012
Status Active
Type General Article