IDT EJTAG System
EJTAG Test Access Port
79RC32438 User Reference Manual
20 - 70
November 4, 2002
Notes
EJTAGBOOT Indication Determines Reset Value of EjtagBrk, ProbTrap and ProbEn
The reset value of the EjtagBrk, ProbTrap, and ProbEn bits follows the setting of the internal EJTAG-
BOOT indication. If the EJTAGBOOT instruction has been given, and the internal EJTAGBOOT indication is
active, then the reset value of the three bits is set (1), otherwise the reset value is clear (0). The results of
setting these bits are:
–
A Debug Interrupt exception is requested right after reset because EjtagBrk is set
–
The debug handler is executed from the EJTAG memory because ProbTrap is set to indicate
debug vector in EJTAG memory at 0xFF20 0200
–
Service of the processor access is indicated because ProbEn is set.
Thus, it is possible to execute the debug handler right after reset, without executing any instructions
from the normal reset handler.
Combinations of ProbTrap and ProbEn
Use of ProbTrap and ProbEn allows independent specification of the debug exception vector location
and availability of EJTAG memory. Behavior for the different combinations is shown in Table 20.49. Note
that not all combinations are allowed.
Bypass Register (TAP Instruction BYPASS, (EJTAG/NORMAL) BOOT, or Unused)
Compliance Level
: Required with EJTAG TAP.
The Bypass register is a one-bit read-only register, which provides a minimum shift path through the
TAP. This register is also defined in IEEE 1149.1. Figure 20.33 shows the format of the Bypass register and
Table 20.50 describes the Bypass register field.
Examples of Use
An example of the TAP operation is shown in Figure 20.34. JTAG_TRST_N is assumed deasserted
high.
ProbTrap
ProbEn
Debug Exception Vector
Processor
Accesses
0
0
Normal memory at 0xBFC0 0480
Not serviced by probe
0
1
Not serviced by probe
1
0
If these two bits are changed to this state, the operation of the processor is
UNDEFINED, indicating that the debug exception vector is in EJTAG memory,
but the probe will not service processor accesses.
1
1
EJTAG memory at 0xFF20 0200
Serviced by Probe
Table 20.49 Combinations of ProbTrap and ProbEn
0
0
32-bit
Processor
Figure 20.33 Bypass Register Format
Fields
Description
Read/
Write
Reset
State
Compli-
ance
Name
Bit
0
0
Ignored on writes; returns zero on reads.
R
0
Required
Table 20.50 Bypass Register Field Description