Datasheet

303
ATmega640/1280/1281/2560/2561
2549K–AVR–01/07
TAP - Test Access Port The JTAG interface is accessed through four of the AVR’s pins. In JTAG terminology,
these pins constitute the Test Access Port – TAP. These pins are:
• 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, the input TAP signals are inter-
nally pulled high and the JTAG is enabled for Boundary-scan and programming. 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
monitored 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 collec-
tors on the reset line are used in the application.
Figure 129. TAP Controller State Diagram
Test-Logic-Reset
Run-Test/Idle
Shift-DR
Exit1-DR
Pause-DR
Exit2-DR
Update-DR
Select-IR Scan
Capture-IR
Shift-IR
Exit1-IR
Pause-IR
Exit2-IR
Update-IR
Select-DR Scan
Capture-DR
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0
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