A minimum of 6 wires is required to connect 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 emulation. 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 figure below 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 resistor should be chosen so that the external circuitry and the AVR do not exceed their maximum ratings (i.e. sinks or sources to much current).
AVR Dragon support emulation of devices placed in a JTAG Chain. When connecting N devices in a JTAG scan chain all devices should connect to TMS and TCK in parallel. The first device should connects it's TDI to the emulator while the TDO should be wired to TDI of the next device up to device N. The last device should connects it's TDO to the emulator.
STK500 does not have a dedicated JTAG interface connector. To connect the AVR Dragon to the STK500 board, the JTAG Probe must be strapped to the appropriate JTAG Port Pins of the target device using a squid cable. Check the target device datasheet for the location of the JTAG pins on the appropriate device. Figure below shows an example on how the pins should be connected for an ATmega32 on the STK500. Remember to remove the reset jumper on the STK500 if the reset pin is going to be controlled from the AVR Dragon.
Note: Add-on cards for the STK500 like e.g. STK501/502 may have a dedicated JTAG connector.
The STK500 JTAG Adapter, that comes with the JTAGICE mkII, can be used to simplify the connection to the STK500 for AVR devices with JTAG that mates with socket SCKT3100A3 and SCKT3000D3 on the STK500.
If the JTAGEN fuse (JTAG Enable) in the target device is un-programmed, the JTAG Interface will be disabled. This fuse cannot be programmed through the JTAG Interface and must therefore be programmed through the ISP Interface or High Voltage Serial or Parallel Interface.