Tip: Remember to include a decoupling capacitor between pin 1 and GND.
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.
Figure 4-3. JTAG Daisy Chain
TMS
TDI
TDO
TCK
target
device
1
target
device
2
target
device
3
programmer /
debugger
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 + Atmel 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.
Tip:
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 (8-bit AVR devices have 4 IR bits)
Atmel Atmel-ICE [USER GUIDE]
Atmel-42330C-Atmel-ICE_User Guide-10/2016
26
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