
Functional Description and Application Information
Background Debug Module (S12SBDMV1)
MM912F634
Freescale Semiconductor
208
Figure 65. Target Acknowledge Pulse (ACK)
Figure 66 shows the ACK handshake protocol in a command level timing diagram. The READ_BYTE instruction is used as an
example. First, the 8-bit instruction opcode is sent by the host, followed by the address of the memory location to be read. The
target BDM decodes the instruction. A bus cycle is grabbed (free or stolen) by the BDM and it executes the READ_BYTE
operation. Having retrieved the data, the BDM issues an ACK pulse to the host controller, indicating that the addressed byte is
ready to be retrieved. After detecting the ACK pulse, the host initiates the byte retrieval process. Note that data is sent in the form
of a word and the host needs to determine which is the appropriate byte based on whether the address was odd or even.
Figure 66. Handshake Protocol at Command Level
NOTE
The only place the BKGD pin can have an electrical conflict is when one side is driving low
and the other side is issuing a speedup pulse (high). Other “highs” are pulled rather than
driven. However, at low rates the time of the speedup pulse can become lengthy and so the
potential conflict time becomes longer as well.
Differently from the normal bit transfer (where the host initiates the transmission), the serial interface ACK handshake pulse is
initiated by the target MCU by issuing a negative edge in the BKGD pin. The hardware handshake protocol in
Figure 65 specifies
the timing when the BKGD pin is being driven, so the host should follow this timing constraint in order to avoid the risk of an
electrical conflict in the BKGD pin.
16 Cycles
BDM Clock
(Target MCU)
Target
Transmits
ACK Pulse
High-impedance
BKGD Pin
Minimum Delay
From the BDM Command
32 Cycles
Earliest
Start of
Next Bit
Speedup Pulse
16th Tick of the
Last Command Bit
High-impedance
READ_BYTE
BDM Issues the
BKGD Pin
Byte Address
BDM Executes the
READ_BYTE Command
Host
Target
Host
Target
BDM Decodes
the Command
ACK Pulse (out of scale)
Host
Target
(2) Bytes are
Retrieved
New BDM
Command