19-12
MCF5249UM
MOTOROLA
Background-Debug Mode (BDM)
corresponds to the data returned by the debug module in response to the previous development system
commands. Command and result transactions are overlapped to minimize latency.
Figure 19-4 Command Sequence Diagram
The cycle in which the command is issued contains the development system command mnemonic (in this
example, “read memory location”). During the same cycle, the debug module responds with either the
low-order results of the previous command or a command complete status (if no results were required).
During the second cycle, the development system supplies the high-order 16 bits of the memory address.
The debug module returns a “not ready” response unless the received command was decoded as
unimplemented, in which case the response data is the illegal command encoding. If an illegal command
response occurs, the development system should retransmit the command.
Note: The “not ready” response can be ignored unless a memory-referencing cycle is in
progress. Otherwise, the debug module can accept a new serial transfer after 32
processor clock periods.
In the third cycle, the development system supplies the low-order 16 bits of a memory address. The debug
module always returns the “not ready” response in this cycle. At the completion of the third cycle, the
debug module initiates a memory read operation. Any serial transfers that begin while the memory access
is in progress return the “not ready” response.
Results are returned in the two serial transfer cycles following the completion of the memory access. The
data transmitted to the debug module during the final transfer is the opcode for the following command. If a
memory or register access is terminated with a bus error, the error status (S=1, DATA=$0001) is returned
in place of the result data.
Next CMD
Read
Memory
Location
“Not Ready”
Next CMD
LS Result
BERR
XXX
“Not Ready”
Next CMD
“Not Ready”
LS Addr
“Not Ready”
XXX
“Illegal”
MS Addr
“Not Ready”
Read (Long)
???
Commands Transmitted to the Debug Module
Command Code Transmitted During This Cycle
Responses from the Debug Module
Results From Previous Command
XXX
MS Result
XXX
High-Order 16 Bits of Memory Address
Low-Order 16 Bits of Memory Address
Non-Serial Related Activity
Sequence Taken If
Operation Has Not
Completed
Next
Mode
Command
Data Unused From
This Transfer
Sequence Taken if Illegal Command
is Received by Debug Module
Sequence Taken if Bus
Error Occurs On
Memory Access
High and Low-Order
16 Bits of Results
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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