Block Guide — S12BDM V4

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Figure 4-4 shows the host receiving a logic 0 from the target. Since the host is asynchronous to the target,

there is up to a one clock-cycle delay from the host-generated falling edge on BKGD to the start of the bit

time as perceived by the target. The host initiates the bit time but the target finishes it. Since the target

wants the host to receive a logic 0, it drives the BKGD pin low for 13 target clock cycles then briefly drives

it high to speed up the rising edge. The host samples the bit level about 10 target clock cycles after starting

the bit time.

Figure 4-4 BDM Target-to-Host Serial Bit Timing (Logic 0)

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 asynchronously related to the bus frequency, when CLKSW = 0, 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 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 4-5). 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 frequency, which in some cases could be very slow

EARLIEST

START OF

NEXT BIT

CLOCK

TARGET SYS.

HOST

DRIVE TO

BKGD PIN

BKGD PIN

PERCEIVED

START OF BIT TIME

10 CYCLES

10 CYCLES

HOST SAMPLES

BKGD PIN

TARGET SYS.

DRIVE AND

SPEEDUP PULSE

SPEEDUP PULSE

HIGH-IMPEDANCE