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The TAP crosses 3 clock domains !

A project log for YGREC8

A byte-wide stripped-down version of the YGREC16 architecture

Yann Guidon / YGDESYann Guidon / YGDES 08/03/2020 at 17:520 Comments

The eXecute module of the TAP connects one domain with no RESET but 2 clocks, to another with one RESET and one clock. This makes it more complex than the others, as hinted by the end of the previous log The TAP's eXecute module.

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For the communication with the FSM, the signal goes through two DFF as shown below :

If the FSM clock is fast enough, the OR can be removed but... you're never too sure ! For example going from RESET to START triggers the reload of the instruction memory, which can take 4K cycles at least.

The first DFF triggers on /WR going up, which is the necessary condition to detect the end of the message, or else the "valid" address could be trigered by enough random data flowing through the shift register. The asynchronous RESET allows the crossing of clock domains, and the clearing always trails the setting by at least one FSM clock cycle, as delayed by the next DFF.

The DFF on the right also re-synchronises the input data so it is valid at the start of each FSM clock cycle. Otherwise the data could arrive late in the cycle and create race conditions and invalid boolean calculations.

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