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IDT EJTAG System
Off-Chip and Probe Interfaces
79RC32438 User Reference Manual
20 - 83
November 4, 2002
Notes
Figure 20.44 Target System Layout for EJTAG Connection
Probe Requirements and Recommendations
A probe connected to the target system at power-up is not allowed to drive the inputs before VIO indi-
cates a stable voltage. JTAG_TRST_N (if present) is then asserted by the target system pull-down resistor
at power-up, whereby a TAP reset is applied through JTAG_TRST_N for TAPs, depending on
JTAG_TRST_N. This step implies that inputs are not driven until the target system is powered up; otherwise
the communication on the TAP might be undefined or damage could occur. At power-down the probe is not
allowed to drive the inputs after VIO has dropped under a certain level (refer to section “Voltage Sense for I/
O (Vcc I/O) Timing” on page 20-79). The RSTN signal is an exception to the above description because it
can be driven low by the probe during power-up.
Hot Plug in of Probe
The probe must not drive any inputs to the target system if it is connected while the system is running
(hot plug). Detection of a stable Vcc I/O from the target system is required before any input is allowed. To
avoid spikes or changes in the input voltage to the target system when the probe is connected, the level of
the signal on the probe must be adjusted to the same level as the signals on the target system. This adjust-
ment can be done with large pull-up/down resistors (in the range of 150 k
) on the probe signals, so the
level of these signals matches the level on the target system shown in Figure 20.44. The specific implemen-
tation of this feature is dependent on the probe, the driver type, etc. used in the probe.
JTAG_TDO Level when Tri-Stated
The probe must apply a pull-up resistor on JTAG_TDO to have a well-defined logical level when
JTAG_TDO on the TAP is tri-stated. The pull-up on the target system ensures the level at hot plug. The size
of the pull-up on the probe is expected to be 1.0 k
or more. The resistor value must be chosen so I
Zstate
is
observed.
RSTN Drive by Open Collector
Drive the RSTN signal with an open-collector (OC) output driver to allow for easy connection of the
RSTN signal in the target system.
Target System PCB
4.0 mm
4.0 mm
3.0 mm
3.0 mm
No components taller than the
base of the pin header should
be placed in the marked area
1