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Chapter 15 Background Debug Module (S12XBDMV2)

MC9S12XDP512 Data Sheet, Rev. 2.21

Freescale Semiconductor

591

The ACK_ENABLE sends an ACK pulse when the command has been completed. This feature could be

used by the host to evaluate if the target supports the hardware handshake protocol. If an ACK pulse is

issued in response to this command, the host knows that the target supports the hardware handshake

protocol. If the target does not support the hardware handshake protocol the ACK pulse is not issued. In

this case, the ACK_ENABLE command is ignored by the target since it is not recognized as a valid

command.

The BACKGROUND command will issue an ACK pulse when the CPU changes from normal to

background mode. The ACK pulse related to this command could be aborted using the SYNC command.

The GO command will issue an ACK pulse when the CPU exits from background mode. The ACK pulse

related to this command could be aborted using the SYNC command.

The GO_UNTIL command is equivalent to a GO command with exception that the ACK pulse, in this

case, is issued when the CPU enters into background mode. This command is an alternative to the GO

command and should be used when the host wants to trace if a breakpoint match occurs and causes the

CPU to enter active background mode. Note that the ACK is issued whenever the CPU enters BDM, which

could be caused by a breakpoint match or by a BGND instruction being executed. The ACK pulse related

to this command could be aborted using the SYNC command.

The TRACE1 command has the related ACK pulse issued when the CPU enters background active mode

after one instruction of the application program is executed. The ACK pulse related to this command could

be aborted using the SYNC command.

15.4.9

SYNC — Request Timed Reference Pulse

The SYNC command is unlike other BDM commands because the host does not necessarily know the

correct communication speed to use for BDM communications until after it has analyzed the response to

the SYNC command. To issue a SYNC command, the host should perform the following steps:

1. Drive the BKGD pin low for at least 128 cycles at the lowest possible BDM serial communication

frequency (the lowest serial communication frequency is determined by the crystal oscillator or the

clock chosen by CLKSW.)

2. Drive BKGD high for a brief speedup pulse to get a fast rise time (this speedup pulse is typically

one cycle of the host clock.)

3. Remove all drive to the BKGD pin so it reverts to high impedance.

4. Listen to the BKGD pin for the sync response pulse.

Upon detecting the SYNC request from the host, the target performs the following steps:

1. Discards any incomplete command received or bit retrieved.

2. Waits for BKGD to return to a logic one.

3. Delays 16 cycles to allow the host to stop driving the high speedup pulse.

4. Drives BKGD low for 128 cycles at the current BDM serial communication frequency.

5. Drives a one-cycle high speedup pulse to force a fast rise time on BKGD.

6. Removes all drive to the BKGD pin so it reverts to high impedance.

The host measures the low time of this 128 cycle SYNC response pulse and determines the correct speed

for subsequent BDM communications. Typically, the host can determine the correct communication speed

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