jtag_tap: update tdo on falling edge of tck

[michg]

Hello Gavin,
updating tdo on rising tck did not not work for me on real hardware, in simulation it was
fine.
Regards
Michael

[atthecodeface]

Thanks Michael.

Sorry it has taken so long to get back - I have been doing a lot of work on FPGAs, and I am going to explode the cdl hardware repo into multiple repositories - all part of a much larger development.

I understand the issue here - that the hardware requires a hold time on tdo from tick that is not met, but it is met if tdo changes on tck falling. Now - is it actually that there needs (in your hardware) to be a hold time after tck falling itself, or that there is a hold requirement on tck rising that is automatically met by delaying tdo until after tck falls?

The correct place for this fix is outside the jtag module, as it is an I/O requirement not a protocol requirement. But I don't think it is wise to just ignore the issue in all the jtag modules, so I might built an external jtag interface module that delays tdo by half a clock.

Thoughts?

[michg]

Hi Gavin,
the Std 1149.1-2001 specifies under 4.5.1 TDO Specification:
Rules: a) Changes in the state of the signal driven through TDO shall occur only on the falling edge of TCK. ..
while changes at the TAP output (TDO) occur on the falling edgeof TCK.
Because of that, I think it is adequate to do this inside the jtag module.
Regards,
Michael