Hope someone can help me, because Atmel's support team don't want to, they log your support request, but no one attend to your problem. I am using an ATMega256A3 device in a vehicle tracking project. I am using 3 of the hardware USARTs in the device. According to the data sheet, all USARTs have a TXD pin, a RXD pin and a XCK pin (external clock). I use the USARTs in asynchronous mode and the internal baud rate generator is used to generate the requested baud rate from the peripheral clock, which is generated from a 14.7456Mhz external crystal. I have used the XCK pin on USART C0 to switch a transistor, to control power to a GSM modem. The problem I have is that the XCK pin on USART C0 seems to be in use by some peripheral function on that I/O pin (pin17). I think that it is USART C0. I need to have full control over the I/O pin (Port C bit 1) It might be one of the other peripheral functions on that pin. I use Timer/Counter C0 also, but only to generate a 32mSec interrupt. All the Capture/Compare pins are disabled. I am debugging the firmware with an JTag ICE MK2 in PDI mode and when I configure PORT C bit 1 as an output and try to switch the I/O high, the JTag ICE get stuck on that instruction and then eventually the execution reset to the reset vector. Can anyone give me any advice on what I do wrong? I have used a number of Mega chips in the past, but this is my first project with an XMega chip, so I am still learning how to use it.