4.1 Connecting to Target through the JTAG Interface

A minimum of six wires is required to connect Atmel AVR Dragon to the target board. These Signals are TCK, TDO, TDI, TMS, VTref, and GND.

Optional line is the nSRST. The nTRST signal is not used, and is reserved for compatibility with other equipment.

nSRST is used to control and monitor the target reset line. This is however not necessary for correct debugging. But if the application code sets the JTD bit in the MCUCSR, the JTAG Interface will be disabled. For the AVR Dragon to reprogram the target AVR, it will need to have control of the Reset Pin.

The figures in Connecting Atmel AVR Dragon to Target Board shows which JTAG lines should be connected to the target AVR to ensure correct operation. To avoid drive contention on the lines it is recommended that series resistors are placed between the JTAG lines and external circuitry. The value of the resistors should be chosen so that the external circuitry and the AVR device do not exceed their maximum ratings (i.e. sinks or sources to much current).