
JTAG Interface
37
March 22, 2006
Preliminary
By default, the internal pull-up resistor on the
TMS
pin is enabled after reset. Changes to the pull-
up resistor settings on GPIO Port C should ensure that the internal pull-up resistor remains
enabled on
PC1/TMS
; otherwise JTAG communication could be lost.
5.2.1.4
Test Data Input (TDI)
The
TDI
pin provides a stream of serial information to the IR chain and the DR chains.
TDI
is
sampled on the rising edge of
TCK
and, depending on the current TAP state and the current
instruction, presents this data to the proper shift register chain. Because the
TDI
pin is sampled on
the rising edge of
TCK
, the
IEEE Standard 1149.1
expects the value on
TDI
to change on the
falling edge of
TCK
.
By default, the internal pull-up resistor on the
TDI
pin is enabled after reset. Changes to the pull-
up resistor settings on GPIO Port C should ensure that the internal pull-up resistor remains
enabled on
PC2/TDI
; otherwise JTAG communication could be lost.
5.2.1.5
Test Data Output (TDO)
The
TDO
pin provides an output stream of serial information from the IR chain or the DR chains.
The value of
TDO
depends on the current TAP state, the current instruction, and the data in the
chain being accessed. In order to save power when the JTAG port is not being used, the
TDO
pin is
placed in an inactive drive state when not actively shifting out data. Because
TDO
can be
connected to the
TDI
of another controller in a daisy-chain configuration, the
IEEE Standard
1149.1
expects the value on
TDO
to change on the falling edge of
TCK
.
By default, the internal pull-up resistor on the
TDO
pin is enabled after reset. This assures that the
pin remains at a constant logic level when the JTAG port is not being used. The internal pull-up
and pull-down resistors can be turned off to save internal power if a High-Z output value is
acceptable during certain TAP controller states.
5.2.2
JTAG TAP Controller
The JTAG TAP controller state machine is shown in Figure 5-2 on page 38. The TAP controller
state machine is reset to the Test-Logic-Reset state on the assertion of a Power-On-Reset (POR)
or the assertion of
TRST
. Asserting the correct sequence on the
TMS
pin allows the JTAG module
to shift in new instructions, shift in data, or idle during extended testing sequences. For detailed
information on the function of the TAP controller and the operations that occur in each state,
please refer to
IEEE Standard 1149.1
.