IDT EJTAG System
On-Chip Interfaces
79RC32438 User Reference Manual
20 - 74
November 4, 2002
Notes
Figure 20.36 Read Processor Access Example
The different probe actions shown in the above figure are described below:
1. The EJTAG Control register is polled to get the indication for a pending PrAcc bit. The PrAcc bit is
written to 1 when polling, in order to prevent a processor access from finishing before being
serviced. The values of PRnW and Psz are saved when PrAcc indicates a pending processor
access.
2. The Address register is read. It contains the address of the load/fetch resulting in the write processor
access, with the three LSBs (64-bit processor) modified to allow size indication together with the
Psz.
3. The Data register is written with the data to return for the load/fetch, resulting in the read processor
access.
4. The PrAcc bit is cleared in order to finish the processor access.
The probe must provide data for the read processor access from the internal EJTAG memory. Note that
the Address register does not contain the direct address from the access, because the three LSBs (64-bit
processor) are modified to indicate the size in conjunction with Psz. Also notice that in this case, there is no
shifting of the data returned for the processor access by writing to the Data register, because a doubleword
is provided. For other accesses, the Data register must be written with a shifted value depending on the
specific access.
On-Chip Interfaces
Optional JTAG_TRST_N Pin
The JTAG_TRST_N signal to the TAP is optional, and need not be provided as a pin on the chip for a
processor implementing the EJTAG TAP. If a JTAG_TRST_N chip pin is not provided, then a TAP reset like
the one provided when JTAG_TRST_N is asserted (low) must be applied to the TAP at power-up, for
example, through a power-up reset circuit on the chip. This power-up TAP reset must be finished after the
time T
VIOrise
(see Figure 20.41). If a JTAG_TRST_N chip pin is provided, then the power-up TAP reset is
applied by a pull-down resistor, because the probe will not drive JTAG_TRST_N at power-up.
Input Buffers with Pull-Up/Down and Output Drivers for Chip Pins
If an input buffer with an integrated pull-up resistor is used for the JTAG_TRST_N chip pin, then its
resistor value must be sufficiently large that it is overruled by the external pull-down resistor on the PCB, so
a well-defined logical level is present on the JTAG_TRST_N pin (refer to section “Electrical Connection” on
page 20-81). Observe the additional rules described in the IEEE Std. 1149.1 specification, if the same TAP
PrAcc
Probe
action
PRnW
Psz
Address
Data
1
1
Address = = 0xF FF203457
Size = 3
2
3
4
1
1
0x0..0 Data =