8-50
MPC7400 RISC Microprocessor Users Manual
Non-Protocol Signal Descriptions
by the test access port (TAP) controller which in turn is controlled by the test mode select
(TMS) input sequence. The scan data is latched in at the rising edge of test clock (TCK).
Test reset (TRST) is an optional JTAG signal which is used in the MPC7400 to reset the
TAP controller asynchronously. The TRST signal assures that the JTAG logic does not
interfere with the normal operation of the device. It is recommended that TRST be asserted
and negated coincident with the assertion of the HRESET signal.
These signals are not used during normal operation. TMS, TDI, and TRST have internal
pull-ups provided; TCK does not. For normal operation, TMS and TDI may be left
unconnected, and TCK must be set high or low. TRST must be asserted sometime during
power-up for JTAG logic initialization. Note that if TRST is tied low, then unnecessary
power is consumed.
8.5.6.1 JTAG Test Clock (TCK)Input
The JTAG test clock (TCK) signal is an input on the MPC7400. Following is the state
meaning for the TCK input signal.
State Meaning
Asserted/NegatedThis input should be driven by a free-running
clock signal. Input signals to the test access port are clocked in on the
rising edge of TCK. Changes to the test access port output signals
occur on the falling edge of TCK. The test logic allows TCK to be
stopped.
8.5.6.2 JTAG Test Data Input (TDI)Input
Following is the state meaning for the TDI input signal.
State Meaning
Asserted/NegatedThe value presented on this signal on the rising
edge of TCK is clocked into the selected JTAG test instruction or
data register.
Note that this input contains an internal pull-up resistor to ensure that
an unterminated input appears as a high signal level to the test logic.
Table 8-7. IEEE Interface Pin Descriptions
Signal Name
Input/Output
Weak Pullup
Provided
IEEE 1149.1a Function
TCK
Input
No
Scan clock
TDI
Input
Yes
Serial scan input signal
TDO
Output
No
Serial scan output signal
TMS
Input
Yes
TAP controller mode signal
TRST
Input
Yes
TAP controller reset