Datasheet

Chapter 15 Background Debug Module (BDMV4)
MC9S12E128 Data Sheet, Rev. 1.07
Freescale Semiconductor 467
Figure 15-12. ACK Abort Procedure at the Command Level
Figure 15-13 shows a conflict between the ACK pulse and the SYNC request pulse. This conflict could
occur if a POD device is connected to the target BKGD pin and the target is already in debug active mode.
Consider that the target CPU is executing a pending BDM command at the exact moment the POD is being
connected to the BKGD pin. In this case, an ACK pulse is issued along with the SYNC command. In this
case, there is an electrical conflict between the ACK speedup pulse and the SYNC pulse. Because this is
not a probable situation, the protocol does not prevent this conflict from happening.
Figure 15-13. ACK Pulse and SYNC Request Conflict
NOTE
This information is being provided so that the MCU integrator will be aware
that such a conflict could eventually occur.
The hardware handshake protocol is enabled by the ACK_ENABLE and disabled by the ACK_DISABLE
BDM commands. This provides backwards compatibility with the existing POD devices which are not
able to execute the hardware handshake protocol. It also allows for new POD devices, that support the
hardware handshake protocol, to freely communicate with the target device. If desired, without the need
for waiting for the ACK pulse.
READ_BYTE READ_STATUSBKGD PIN MEMORY ADDRESS NEW BDM COMMAND
NEW BDM COMMAND
HOST TARGET
HOST TARGET
HOST TARGET
SYNC RESPONSE
FROM THE TARGET
(OUT OF SCALE)
BDM DECODE
AND STARTS TO EXECUTES
THE READ_BYTE CMD
READ_BYTE CMD IS ABORTED
BY THE SYNC REQUEST
(OUT OF SCALE)
BDM CLOCK
(TARGET MCU)
TARGET MCU
DRIVES TO
BKGD PIN
BKGD PIN
16 CYCLES
SPEEDUP PULSE
HIGH-IMPEDANCE
HOST
DRIVES SYNC
TO BKGD PIN
ACK PULSE
HOST SYNC REQUEST PULSE
AT LEAST 128 CYCLES
ELECTRICAL CONFLICT
HOST AND
TARGET DRIVE
TO BKGD PIN