ATmega64A datasheet(253/392 Pages) ATMEL

253 / 392 page

253

8160C–AVR–07/09

ATmega64A

• TMS: Test mode select. This pin is used for navigating through the TAP-controller state

machine.

• TCK: Test clock. JTAG operation is synchronous to TCK.

• TDI: Test Data In. Serial input data to be shifted in to the Instruction Register or Data Register

(Scan Chains).

• TDO: Test Data Out. Serial output data from Instruction Register or Data Register.

The IEEE std. 1149.1 also specifies an optional TAP signal; TRST – Test ReSeT – which is not

provided.

When the JTAGEN fuse is unprogrammed, these four TAP pins are normal port pins and the

TAP controller is in reset. When programmed and the JTD bit in MCUCSR is cleared, the TAP

input signals are internally pulled high and the JTAG is enabled for Boundary-scan and program-

ming. In this case, the TAP output pin (TDO) is left floating in states where the JTAG TAP

controller is not shifting data, and must therefore be connected to a pull-up resistor or other

hardware having pull-ups (for instance the TDI-input of the next device in the scan chain). The

device is shipped with this fuse programmed.

For the On-chip Debug system, in addition to the JTAG interface pins, the RESET pin is moni-

tored by the debugger to be able to detect External Reset sources. The debugger can also pull

the RESET pin low to reset the whole system, assuming only open collectors on the reset line

are used in the application.

ATmega64A Datasheet(PDF) 253 Page - ATMEL Corporation