MM912F634CV2AP datasheet(205/335 Pages) FREESCALE

205 / 335 page

Functional Description and Application Information

Background Debug Module (S12SBDMV1)

MM912F634

Freescale Semiconductor

205

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

MM912F634CV2AP Datasheet(PDF) 205 Page - Freescale Semiconductor, Inc

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