Background Debug Module (S12SBDMV1)
5.4.7
Serial Interface Hardware Handshake Protocol
BDM commands that require CPU execution are ultimately treated at the MCU bus rate. Since the BDM
clock source can be modified when changing the settings for the VCO frequency (CPMUSYNR), it is very
helpful to provide a handshake protocol in which the host could determine when an issued command is
executed by the CPU. The BDM clock frequency is always VCO frequency divided by 8. The alternative
is to always wait the amount of time equal to the appropriate number of cycles at the slowest possible rate
the clock could be running. This sub-section will describe the hardware handshake protocol.
The hardware handshake protocol signals to the host controller when an issued command was successfully
executed by the target. This protocol is implemented by a 16 serial clock cycle low pulse followed by a
brief speedup pulse in the BKGD pin. This pulse is generated by the target MCU when a command, issued
by the host, has been successfully executed (see Figure 5-10). This pulse is referred to as the ACK pulse.
After the ACK pulse has finished: the host can start the bit retrieval if the last issued command was a read
command, or start a new command if the last command was a write command or a control command
(BACKGROUND, GO, GO_UNTIL or TRACE1). The ACK pulse is not issued earlier than 32 serial clock
cycles after the BDM command was issued. The end of the BDM command is assumed to be the 16th tick
of the last bit. This minimum delay assures enough time for the host to perceive the ACK pulse. Note also
that, there is no upper limit for the delay between the command and the related ACK pulse, since the
command execution depends upon the CPU bus, which in some cases could be very slow due to long
accesses taking place.This protocol allows a great flexibility for the POD designers, since it does not rely
on any accurate time measurement or short response time to any event in the serial communication.
BDM Clock
(Target MCU)
16 Cycles
Target
Transmits
ACK Pulse
High-Impedance
32 Cycles
High-Impedance
Speedup Pulse
Minimum Delay
From the BDM Command
BKGD Pin
Earliest
Start of
Next Bit
16th Tick of the
Last Command Bit
Figure 5-10. Target Acknowledge Pulse (ACK)
NOTE
If the ACK pulse was issued by the target, the host assumes the previous
command was executed. If the CPU enters wait or stop prior to executing a
hardware command, the ACK pulse will not be issued meaning that the
BDM command was not executed. After entering wait or stop mode, the
BDM command is no longer pending.
S12P-Family Reference Manual, Rev. 1.13
146
Freescale Semiconductor
FREESCALE [ Freescale ]
