
LM3S101 Data Sheet
March 22, 2006
36
Preliminary
5.2.1
JTAG Interface Pins
The JTAG interface consists of five standard pins:
TRST
,
TCK
,
TMS
,
TDI
, and
TDO
. These pins and
their associated reset state are given in Table 5-1. Detailed information on each pin follows.
5.2.1.1
Test Reset Input (TRST)
The
TRST
pin is an asynchronous active Low input signal for initializing and resetting the JTAG
TAP controller and associated JTAG circuitry. When
TRST
is asserted, the TAP controller resets to
the Test-Logic-Reset state and remains there while
TRST
is asserted. When the TAP controller
enters the Test-Logic-Reset state, the Instruction Register (IR) resets to the default instruction,
IDCODE.
By default, the internal pull-up resistor on the
TRST
pin is enabled after reset. Changes to the pull-
up resistor settings on GPIO Port B should ensure that the internal pull-up resistor remains
enabled on
PB7
/
TRST
; otherwise JTAG communication could be lost.
5.2.1.2
Test Clock Input (TCK)
The
TCK
pin is the clock for the JTAG module. This clock is provided so the test logic can operate
independently of any other system clocks. In addition, it ensures that multiple JTAG TAP
controllers that are daisy-chained together can synchronously communicate serial test data
between components. During normal operation,
TCK
is driven by a free-running clock with a
nominal 50% duty cycle. When necessary,
TCK
can be stopped at 0 or 1 for extended periods of
time. While
TCK
is stopped at 0 or 1, the state of the TAP controller will not change and data in the
JTAG instruction and data registers will not be lost.
By default, the internal pull-up resistor on the
TCK
pin is enabled after reset. This assures that no
clocking occurs if the pin is not driven from an external source. The internal pull-up and pull-down
resistors can be turned off to save internal power as long as the
TCK
pin is constantly being driven
by an external source.
5.2.1.3
Test Mode Select (TMS)
The
TMS
pin selects the next state of the JTAG TAP controller.
TMS
is sampled on the rising edge
of
TCK
. Depending on the current TAP state and the sampled value of
TMS
, the next state is
entered. Because the
TMS
pin is sampled on the rising edge of
TCK
, the
IEEE Standard 1149.1
expects the value on
TMS
to change on the falling edge of
TCK
.
Holding
TMS
high for five consecutive
TCK
cycles drives the TAP controller state machine to the
Test-Logic-Reset state. When the TAP controller enters the Test-Logic-Reset state, the JTAG
Instruction Register (IR) resets to the default instruction, IDCODE. Therefore, this sequence can
be used as a reset mechanism, similar to asserting
TRST
. The JTAG Test Access Port state
machine can be seen in its entirety in Figure 5-2.
Table 5-1.
JTAG Port Pins Reset State
Pin Name
Data
Direction
Internal
Pull-Up
Internal
Pull-Down
Drive
Strength
Drive Value
TRST
Input
Enabled
Disabled
N/A
N/A
TCK
Input
Enabled
Disabled
N/A
N/A
TMS
Input
Enabled
Disabled
N/A
N/A
TDI
Input
Enabled
Disabled
N/A
N/A
TDO
Output
Enabled
Disabled
2-mA driver
High-Z