Xmod FTDI JTAG Adapter: Why the clock speed limit?

[offroad]

Hi,

I'm using the XMOD FTDI JTAG adapter together with the TE0725 board.
For some reason, the maximum clock speed over JTAG is only half of what I get with other FTDI boards:
On the FTDI chip, I have to use clock divider "1" or JTAG IO fails. When I use a plain FTDI breakout board (e.g. mikroe's FTDI click module), clock divider "0" works reliably.
I've tried independent FPGA- and XMOD boards, so we can rule out the hardware, soldering etc. And even reading IDCODE, which is just a handful of bits, fails at full speed.

To put this into perspective, most users won't notice the difference. Uploading a bitstream is near-instantaneous also with the slower clock.
Now my application is speed-critical. Going from divider 1 to 0 gives almost twice the data rate (measured), and it would be nice if I could run my regular control code already through the debug header.

Is it known why the FTDI chip's fastest clock setting doesn't work with this board?
There is the Lattice FPGA in-between. I haven't yet looked what's inside, but I'd guess it just makes a combinational connection between FTDI chip and header (EDIT: I checked - it does, and the Lattice data sheet shows 150 MHz for most IO primitives, which is several times what the FTDI chip can deliver). Is there anything that could be tweaked, e.g. drive strength of the Lattice FPGA outputs?

[offroad]

Thinking some more about this: Could it be a power supply issue?
I'm running the FPGA from the XMOD module's regulator (which isn't recommended but my design estimates close to 0 power). With clock divider 1 I have transferred many gigabytes of data through JTAG to block RAM and back without ever observing a single bit error.

[Antti Lukats]

1) please do not power from XMOD, really this may lead to any unexpected problems.

2) the DELAY in the XMOD does introduce some delay and skew, so that may be the problem, but it does surprise us a bit, normally Vivado has no problems with 725 and 790 as JTAG.
Some tweaking could be applied if it really is needed or makes difference.

[offroad]

Hi Antti,

thanks for the reply.
For clarification: Vivado does have no problems with it, but it probably runs at lower clock speed anyway.
I am using FTDI's own JTAG library, where I can increase the rate to the limits of the FT2232H chip.

[Antti Lukats]

Hi

I'see I did sometimes the same, but with the FTDI MPSSE you can tune the clock edge.. so with my own FTDI MPSSE software I also have seen problems