Restrictions
MOTOROLA
IEEE 1149.1 Test Access Port (JTAG)
20-9
20.4.4
JTAG BYPASS REGISTER
The MCF5249 includes an IEEE 1149.1A-compliant bypass register, which creates a single bit shift
register path from TDI to the bypass register to TDO when the BYPASS instruction is selected.
20.5
RESTRICTIONS
The test logic is implemented using static logic design, and TCK can be stopped in either a high or low
state without loss of data. The system logic, however, operates on a different system clock which is not
synchronized to TCK internally. Any mixed operation requiring the use of 1149.1 test logic in conjunction
with system functional logic that uses both clocks, must have coordination and synchronization of these
clocks done externally to the MCF5249.
20.6
DISABLING IEEE 1149.1A STANDARD OPERATION
There are two ways to use the MCF5249 without the IEEE 1149.1A test logic being active:
1. Nonuse of the JTAG test logic by either nontermination (disconnection) or intentional fixing of TAP
logic values.
2. Intentional disabling of the JTAG test logic by setting test[3:0]= 0001 (entering Debug mode)
There are several considerations that must be addressed if the IEEE 1149.1A logic is not going to be used
once the MCF5249 is assembled onto a board.
The prime consideration is to ensure that the IEEE 1149.1A test logic remains transparent and benign to
the system logic during functional operation. This requires the minimum of either connecting the TRST pin
to logic 0, or connecting the TCK clock pin to a clock source that will supply five rising edges and the falling
edge after the fifth rising edge, to ensure that the part enters the test-logic-reset state. The recommended
solution is to connect TRST to logic 0.
Another consideration is that the TCK pin does not have an internal pullup as is required on the TMS, TDI,
and TRST pins; therefore, it should not be left unterminated to preclude mid-level input values. Figure 20-3 shows pin values recommended for disabling JTAG with the MCF5249 in JTAG mode.
.
Figure 20-3 Disabling JTAG in JTAG Mode
A second method of using the MCF5249 without the IEEE 1149.1A logic being active is to select Debug
mode by setting test[3:0]= 0001. The IEEE 1149.1A test controller is now placed in the test-logic-reset
TDI/DSI
TCK
VDD
TRST/DSCLK
TMS/BKPT
NOTE: test[3:0] SET TO ‘ 0001’ ALLOWS JTAG MODE.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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