4.2.2.2 JTAG Daisy-Chaining

The JTAG interface allows for several devices to be connected to a single interface in a daisy-chain configuration. The target devices must all be powered by the same supply voltage, share a common ground node, and must be connected as shown in the figure below.

When connecting devices in a daisy-chain, the following points must be considered:

• All devices must share a common ground, connected to GND on the Atmel-ICE probe

• All devices must be operating on the same target voltage. VTG on the Atmel-ICE must be connected to this voltage.

• TMS and TCK are connected in parallel; TDI and TDO are connected in a serial chain.

• nSRST on the Atmel-ICE probe must be connected to RESET on the devices if any of the devices in the chain disables its JTAG port

• “Devices before” refers to the number of JTAG devices that the TDI signal has to pass through in the daisy-chain before reaching the target device. Similarly, “devices after” is the number of devices that the signal has to pass through after the target device before reaching the Atmel-ICE TDO pin.

• Instruction bits “before” and “after” refers to the total sum of all JTAG devices’ instruction register lengths, which are connected before and after the target device in the daisy-chain

• The total IR length (instruction bits before + Microchip target device IR length + instruction bits after) is limited to a maximum of 256 bits. The number of devices in the chain is limited to 15 before and 15 after.

To connect to the Microchip AVR XMEGA device, the daisy-chain settings are:

• Devices before: 1

• Devices after: 1

• Instruction bits before: 4 (8-bit AVR devices have 4 IR bits)

• Instruction bits after: 5 (32-bit AVR devices have 5 IR bits)