mc9s12vr48 Freescale Semiconductor, Inc, mc9s12vr48 Datasheet - Page 191

• Manufacturer Part Number: mc9s12vr48
• Description: S12 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

Figure 5-11
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.
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
should follow this timing constraint in order to avoid the risk of an electrical conflict in the BKGD pin.
The ACK handshake protocol does not support nested ACK pulses. If a BDM command is not
acknowledge by an ACK pulse, the host needs to abort the pending command first in order to be able to
issue a new BDM command. When the CPU enters wait or stop while the host issues a hardware command
(e.g., WRITE_BYTE), the target discards the incoming command due to the wait or stop being detected.
Therefore, the command is not acknowledged by the target, which means that the ACK pulse will not be
issued in this case. After a certain time the host (not aware of stop or wait) should decide to abort any
possible pending ACK pulse in order to be sure a new command can be issued. Therefore, the protocol
provides a mechanism in which a command, and its corresponding ACK, can be aborted.
Freescale Semiconductor
BKGD Pin
shows the ACK handshake protocol in a command level timing diagram. The READ_BYTE
READ_BYTE
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.
Host
Figure 5-10
Figure 5-11. Handshake Protocol at Command Level
Byte Address
Target
MC9S12VR Family Reference Manual, Rev. 2.2
Preliminary - Subject to Change Without Notice
BDM Decodes
the Command
specifies the timing when the BKGD pin is being driven, so the host
NOTE
READ_BYTE Command
BDM Executes the
Target
BDM Issues the
ACK Pulse (out of scale)
(2) Bytes are
Retrieved
Background Debug Module (S12SBDMV1)
Host
Host
Command
New BDM
Target
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