Is the flash device close on the PCB to the CPU?
Which quad flash device are you using? If not an XMOS factory supported flash and to rule out the flash device itself, can you source such a device for similar testing? A few years ago we did a review of the flash devices with the XMOS processors and found assorted pin drive issues across vendors. Back then, found that Spansion (aka Cypress now) had excellent signal quality for our rather long but shielded logic analyzer probes (sold by Zeroplus). Winbond kept failing with value mismatches under the same conditions and cables. Then Winbond sampled us another die of the flash memory but this new device offered a pin drive strength register. We were testing SPI and QSPI modes of the flash devices.
For giggles, consider to source and test out the QSPI FRAM device from Cypress:
http://www.cypress.com/documentation/ap ... qspi-f-ram
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