mc9s12e256 Freescale Semiconductor, Inc, mc9s12e256 Datasheet - Page 467

• Manufacturer Part Number: mc9s12e256
• Description: Hcs12 Microcontrollers 16-bit Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

If an interrupt is pending when a TRACE1 command is issued, the interrupt stacking operation occurs but
no user instruction is executed. Upon return to standard BDM firmware execution, the program counter
points to the first instruction in the interrupt service routine.
15.4.11 Instruction Tagging
The instruction queue and cycle-by-cycle CPU activity are reconstructible in real time or from trace
history that is captured by a logic analyzer. However, the reconstructed queue cannot be used to stop the
CPU at a specific instruction. This is because execution already has begun by the time an operation is
visible outside the system. A separate instruction tagging mechanism is provided for this purpose.
The tag follows program information as it advances through the instruction queue. When a tagged
instruction reaches the head of the queue, the CPU enters active BDM rather than executing the instruction.
Executing the BDM TAGGO command configures two system pins for tagging. The TAGLO signal shares
a pin with the LSTRB signal, and the TAGHI signal shares a pin with the BKGD signal.
Table 15-7
one pin does not affect the function of the other. The presence of logic level 0 on either pin at the fall of
the external clock (ECLK) performs the indicated function. High tagging is allowed in all modes. Low
tagging is allowed only when low strobe is enabled (LSTRB is allowed only in wide expanded modes and
emulation expanded narrow mode).
15.4.12 Serial Communication Time-Out
The host initiates a host-to-target serial transmission by generating a falling edge on the BKGD pin. If
BKGD is kept low for more than 128 target clock cycles, the target understands that a SYNC command
was issued. In this case, the target will keep waiting for a rising edge on BKGD in order to answer the
SYNC request pulse. If the rising edge is not detected, the target will keep waiting forever without any
time-out limit.
Consider now the case where the host returns BKGD to logic one before 128 cycles. This is interpreted as
a valid bit transmission, and not as a SYNC request. The target will keep waiting for another falling edge
marking the start of a new bit. If, however, a new falling edge is not detected by the target within 512 clock
cycles since the last falling edge, a time-out occurs and the current command is discarded without affecting
memory or the operating mode of the MCU. This is referred to as a soft-reset.
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shows the functions of the two tagging pins. The pins operate independently, that is the state of
Tagging is disabled when BDM becomes active and BDM serial commands
are not processed while tagging is active.
TAGHI
1
1
0
0
MC9S12E256 Data Sheet, Rev. 1.08
Table 15-7. Tag Pin Function
TAGLO
NOTE
1
0
1
0
Both bytes
High byte
Low byte
No tag
Tag
Chapter 15 Background Debug Module (BDMV4)
467