instruction cache invalidation breaks timing on teensy4

[v0lker]

• Arduino board: teensy 4.1

• Arduino IDE version (found in Arduino -> About Arduino menu): not installed / not relevant

REPRO STEPS BELOW

• apply patch from cope with I$ miss on teensy #405

• define REPRODUCE_TEENSY_TIMING in Adafruit_NeoPixel.cpp and enable choice of reproducing frequency in (i was driving 2 strips, one after the other and by changing the condition between if (!(ct & 1)) { , if (ct & 1) { or if (true) {, i can break the timing on either the second, first or both strips.

• the desired colour is orange (RGB 0xee4602, GRB 0x46ee02)

• the colour when the first bit is dropped is green (logic analyser shows 0x8ddc04 == (0x46ee02 << 1))

here are the traces:

proposed fix is in PR #405

[v0lker]

@KurtE , @PaulStoffregen as you were previously involved in this, you might have some thoughts?