AVR® JTAG Pinout

When designing an application PCB, which includes a Microchip AVR MCU with the JTAG interface, it is recommended to use the pinout as shown in the figure below. Both 100-mil and 50-mil variants of this pinout are supported, depending on the cabling and adapters included with the particular kit.

Figure 1. AVR® JTAG Header Pinout

Table 1. AVR® JTAG Pin Description
Name Pin Description
TCK 1 Test Clock (clock signal from the Atmel-ICE into the target device)
TMS 5 Test Mode Select (control signal from the Atmel-ICE into the target device)
TDI 9 Test Data In (data transmitted from the Atmel-ICE into the target device)
TDO 3 Test Data Out (data transmitted from the target device into the Atmel-ICE)
nTRST 8 Test Reset (optional, only on some AVR devices). Used to reset the JTAG TAP controller
nSRST 6 Reset (optional). Used to reset the target device. Connecting this pin is recommended since it allows the Atmel-ICE to hold the target device in a Reset state, which can be essential to debugging in certain scenarios.
VTG 4 Target voltage reference. The Atmel-ICE samples the target voltage on this pin to power the level converters correctly. The Atmel-ICE draws less than 3 mA from this pin in debugWIRE mode and less than 1 mA in other modes.
GND 2, 10 Ground. Both must be connected to ensure that the Atmel-ICE and the target device share the same ground reference.
Tip: Remember to include a decoupling capacitor between pin 4 and GND.