Debug Options available
MPC5746R Hardware Design Guide, Rev. 1
NXP Semiconductors 31
to crosstalk from high-current or high-speed signals), optional external pull resistors as shown in Table 16
can be added to ensure proper operation under all conditions.
In addition to the pull-up and pull-down resistors, some systems may want to use buffers between the
JTAG connector inputs (JCOMP, TDI, TDO, TMS, EVTI, EVTO, PORST and RESET) and the MCU.
This will prevent over-voltage conditions from causing damage to the MCU pins. Normal systems should
not require this circuitry, but it is helpful in systems that can be exposed to improper connections that
provide voltages that are outside the operating conditions of the MCU. A common circuit for this
application is the Texas Instruments SN74CBTLV38615. This device is a bus switch that implements a
bidirectional interface between two terminals with less than 5
Ω of resistance. It should be powered by the
same supply that powers the debug port. The device enable pin should be connected to ground for the
interface to be enabled whenever the debug port on the MCU is powered. This circuit provides a high
impedance to the tool when the debug port is powered off.
Table 16. Optional External Pull Ups/Pull Downs
JTAG Signal Component Description
JCOMP
10 kΩ pull-down resistor Holds debug port in reset and prevents
any debug commands from interfering
with the normal operation of the MCU.
PORST
4.7 kΩ pull-up resistor The PORST input should be driven from
an open collector output; therefore, it
requires a pull-up resistor for the MCU.
RESET
4.7 kΩ pull-up resistor The RESET input should be driven from
an open collector output; therefore, it
requires a pull-up resistor for the MCU.
EVTI
10 kΩ pull-up resistor A pull-up resistor prevents debug mode
from being forced after reset if debug
mode is enabled (JCOMP = high). It also
prevents breakpoints from being forced if
debug mode is enabled.
NOTE: In almost all situations, a resistor
is not required on this signal.
TCK
1
1
TCK isolation may be required if using both a JTAG connector and a Nexus Aurora trace connector in the
target system to reduce reflections. Selection of TCK to either connector should be supported. On the NXP
Trace Adapter board, by default, the debug connectors signals are not connected to the target system and
are already isolated.
series 0 Ω resistor A series resistor may be required on TCK
to allow the TCK signal in the target
system to be disconnected for signal
integrity if TCK is connected to multiple
connectors.
To Next Page
Loading...
