IDT EJTAG System
Off-Chip and Probe Interfaces
79RC32438 User Reference Manual
20 - 82
November 4, 2002
Notes
Figure 20.43 Target System Electrical EJTAG Connection
In Figure 20.43, the pull-up resistors for JTAG_TCK, EJTAG_TMS, JTAG_TDI, and RSTN, the pull-down
resistor for JTAG_TRST_N, and the series resistor for JTAG_TDO must be adjusted to the specific design.
However, the recommended pull-up/down resistor is 1.0 k
, because a low value reduces crosstalk on the
cable to the connector, allowing higher JTAG_TCK frequencies. A typical value for the series resistor is
33
. Recommended resistor values have ± 5% tolerance.
The IEEE 1149.1 specification requires that the EJTAG TAP controller is reset at power-up, which can
occur through a pull-down resistor on JTAG_TRST_N if the probe is not connected. However, on-chip pull-
up resistors are implemented on the RC32438 due to an IEEE 1149.1 requirement. Having on-chip pull-up
and external pull-down resistors for the JTAG_TRST_N signal requires special care in the design to ensure
that a valid logical level is provided to JTAG_TRST_N, for example, using a small external pull-down
resistor to ensure this level overrides the on-chip pull-up. An alternative is to use an active power-up reset
circuit for JTAG_TRST_N, which drives JTAG_TRST_N low only at power-up and then holds
JTAG_TRST_N high afterwards with a pull-up resistor.
The pull-up resistor on JTAG_TDO must ensure that the JTAG_TDO level is high when no probe is
connected and the JTAG_TDO output is tri-stated. This requirement allows reliable connection of the probe
if it is hooked-up when the power is already on (hot plug). The pull-up resistor value of around 47 k
should
be sufficient.
Optional diodes to protect against overshoot and undershoot voltage can be provided on the signals to
the chip with EJTAG.
The RST* signal must have a pull-up resistor because it is controlled by an open-collector (OC) driver in
the probe, and thus is actively pulled low only. The pull-up resistor is responsible for the high value when
not driven by the probe. The input on the target system reset circuit must be able to accept the rise time
when the pull-up resistor charges the C
Target
and C
Probe
capacitance to a high logical level.
Vcc I/O must connect to a voltage reference that drops rapidly to below T
VIOactive
when the target
system loses power, even with the capacitive load of C
Probe
. The probe can thus detect the lost power
condition.
Layout Considerations
Layout around the pin connector on the target system must provide for sufficient clearance for the probe
to connect. Figure 20.44 shows the recommended clearance. Place the connector at the edge of the PCB.
Avoid tall components around the connector to allow for easy access.
GND
GND
GND
GND
GND
1
TRST*
TDI
TDO
TMS
TCK
RST*
DINT
no connect
JTAG_TRST_N
JTAG_TDI
JTAG_TDO
EJTAG_TMS
JTAG_TCK
GND
VDD
GND
VccIO voltage
reference
P
P
Series-res.
Reset (soft/hard)
Target System
Reset Circuit
P
Other reset
sources
VccIO
RC32438