Hello,
I have a Spartan 3A starter board (HW-SPAR3A-SK-UNI-G) with the XC3S700A FPGA. I’d like to use its on-board USB JTAG interface to connect with external devices, namely the Spartan 3E XC3S500E-PQG208 and the XC95108 CPLD. In iMPACT, a scan of the JTAG chain results in a random number of devices being detected, which are labeled “unknown bypass,” if the chain scan stops at all. The external chips are confirmed to work stand-alone in another jig.
JTAG Chain Configuration:
- JTSEL to +5V – to inhibit on-board loopback
- TMS, TCK in parallel
- [eval J23 open >> eval FPGA >> eval PROM XCF04S >> eval J17 FX2 expander connector, B3] >> TDI of external device, TDO >> [eval J17 FX2 expander connector, A5]
Test Results:
- connecting the TDI and TDO from the eval board cause normal operation
- swapping TDI and TDO in exasperation lead to the JTAG chain failing with “error message” (yes, chips tested ok afterward)
Thoughtful suggestions appreciated.
