Make the story straight. The Atmel ISP connector combines SPI bus and RESET signal. SPI is used for downloading program, and RESET implements Atmel’s “debugWire” on-chip debug system. debugWire OCD is a very fancy protocol that uses only one RESET pin to debug. ATmega328p supports debugWire, and hence the ISP connector on Arduino UNO can be used for debugging. Arduino Micro uses a newer ATmega32U4 chip. Surprisingly debugWire is missing from the product datasheet, instead JTAG is listed. JTAG uses 4 I/O pins,  from Arduino Micro schematic we can see the JTAG pins (TDI/TDO/TMS/TCK) are multiplexed with analog pin A0-A3. It appears I have to sacrifice these analog pins for the JTAG interface.

JTAGICE3 comes with a 10-pin JTAG header, with the definition as follows:

All I need to do is to make an adapter connecting these pins:

The actual construction is done on a perf board. I made it Arduino “Shield” type so they can be simply stacked together.

I admit it looks quite dangerous when stacked up

I hook up JTAGICE3 and choose JTAG interface in Atmel Studio. And no surprise. Murphy gets me everytime! “[ERROR] No JTAG devices detected.” is the only reply I get. I Scratch my head several hours then to realize how stupid I were. Since the pins PF4-PF7 have been used as analog inputs for Arduino, how can JTAG establish connection with these pins! And the resolution is simple: connect using ISP header and enable the JTAGEN fuse bit. Now I feel the usefulness of ISP connector.

I can finally talk to ATmega32U4 using JTAG and do some debugging. The coming year will be interesting

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