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 Figure 1.
When connecting devices in a daisy-chain, the following points must be considered:
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All devices must share a common ground, connected to GND on the Atmel AVR Dragon probe
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All devices must be operating on the same target voltage. VTG on the AVR Dragon must be connected to this voltage.
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TMS and TCK are connected in parallel; TDI and TDO are connected in a serial chain.
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nSRST on the AVR Dragon probe must be connected to RESET on the devices if any one of the devices in the chain disables its JTAG port
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"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 AVR Dragon TDO pin.
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"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.
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The total IR length (instruction bits before + AVR 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.
Daisy chaining example: TDI -> ATmega1280 -> ATxmega128A1 -> ATUC3A0512 -> TDO
In order to connect to the Atmel AVR XMEGA device, the daisy chain settings are:
Devices before: 1
Devices after: 1
Instruction bits before: 4 (AVR 8-bit microcontrollers have 4 IR bits)
Instruction bits before: 5 (AVR 32-bit microcontrollers have 5 IR bits)