Trying out the ATMega16U2 for the first time. Having issues with getting the Atmel ICE to read the chip though. Very strange I haven't had this for a very long time with other chips. Normally if this happens its because some chips have ISP on pins other than the usual SPI port, but that's not this case. I'm 99% sure I have the wiring right (always that 1% though). Attached is my circuit, and a screenshot of the Mega16U2 datasheet for serial programming.
I have tried several PCBs all the same, tried adding 100nF across the reset line, tried manually injecting an 8MHz clock on the XTAL1 line in case the internal RC isn't working (they are factory chips so should be RC by default), also tried a special jumper cable with MOSI/MISO swapped in case I stuffed that up. I also tried removing U2 which also sits on the SPI lines, and R2 as well in case the pullups were the issue.
Any other thoughts? I'm sure it's something stupid I've missed.
Cheers - Matt