XMEGAA Datasheet, PDF (312/375 Pages) ATMEL Corporation – 8-bit XMEGAA Microcontroller

XMEGA A
When using the JTAG interface for Boundary-scan, the JTAG TCK clock frequency can be
higher than the internal device frequency. The System Clock in the device is not required for
Boundary-scan.
26.3
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. 1149.1-2001 also specifies an optional TAP signal; TRST - Test ReSeT. This is
not available.
When the JTAGEN Fuse is unprogrammed or the JTAG Disable bit is set the JTAG interface is
disabled. The four TAP pins are normal port pins and the TAP controller is in reset. When
enabled, the input TAP signals are internally pulled high and the JTAG is enabled for Boundary-
scan operations.
Figure 26-1. TAP Controller state diagram
8077B–AVR–06/08
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