Atmel AT32UC3A3128, Manual

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940
32072H–AVR32–10/2012
AT32UC3A3
Apply the TMS sequence 1, 1, 0 to re-enter the Run-Test/Idle state. If the selected Data Register
has a latched parallel-output, the latching takes place in the Update-DR state. The Exit-DR,
Pause-DR, and Exit2-DR states are only used for navigating the state machine.
As shown in the state diagram, the Run-Test/Idle state need not be entered between selecting
JTAG instruction and using Data Registers.
35.4.9 Boundary-scan
The boundary-scan chain has the capability of driving and observing the logic levels on the digi-
tal I/O pins, as well as the boundary between digital and analog logic for analog circuitry having
off-chip connections. At system level, all ICs having JTAG capabilities are connected serially by
the TDI/TDO signals to form a long shift register. An external controller sets up the devices to
drive values at their output pins, and observe the input values received from other devices. The
controller compares the received data with the expected result. In this way, boundary-scan pro-
vides a mechanism for testing interconnections and integrity of components on Printed Circuits
Boards by using the 4 TAP signals only.
The four IEEE 1149.1 defined mandatory JTAG instructions IDCODE, BYPASS, SAMPLE/PRE-
LOAD, and EXTEST can be used for testing the Printed Circuit Board. Initial scanning of the
data register path will show the ID-code of the device, since IDCODE is the default JTAG
instruction. It may be desirable to have the 32-bit AVR device in reset during test mode. If not
reset, inputs to the device may be determined by the scan operations, and the internal software
may be in an undetermined state when exiting the test mode. If needed, the BYPASS instruction
can be issued to make the shortest possible scan chain through the device. The device can be
set in the reset state either by pulling the external RESETn pin low, or issuing the AVR_RESET
instruction with appropriate setting of the Reset Data Register.
The EXTEST instruction is used for sampling external pins and loading output pins with data.
The data from the output latch will be driven out on the pins as soon as the EXTEST instruction
is loaded into the JTAG IR-register. Therefore, the SAMPLE/PRELOAD should also be used for
setting initial values to the scan ring, to avoid damaging the board when issuing the EXTEST
instruction for the first time. SAMPLE/PRELOAD can also be used for taking a snapshot of the
external pins during normal operation of the part.
When using the JTAG Interface for boundary-scan, the JTAG TCK clock is independent of the
internal chip clock. The internal chip clock is not required to run during boundary-scan
operations.
NOTE: For pins connected to 5V lines care should be taken to not drive the pins to a logic one
using boundary-scan, as this will create a current flowing from the 3,3V driver to the 5V pull-up
on the line. Optionally a series resistor can be added between the line and the pin to reduce the
current.
Details about the boundary-scan chain can be found in the BSDL file for the device. This can be
found on the Atmel website.
35.4.10 Service Access Bus
The AVR32 architecture offers a common interface for access to On-Chip Debug, programming,
and test functions. These are mapped on a common bus called the Service Access Bus (SAB),
which is linked to the JTAG through a bus master module, which also handles synchronization
between the TCK and SAB clocks.