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Debug Support
MCF5272 ColdFire
®
Integrated Microprocessor User’s Manual, Rev. 3
Freescale Semiconductor
5-17
5.5.2
BDM Serial Interface
When the CPU is halted and PST reflects the halt status, the development system can send unrestricted
commands to the debug module. The debug module implements a synchronous protocol using two inputs
(DSCLK and DSI) and one output (DSO), where DSO is specified as a delay relative to the rising edge of
the processor clock. See
. The development system serves as the serial communication channel
master and must generate DSCLK.
The serial channel operates at a frequency from DC to 1/5 of the PSTCLK frequency. The channel uses
full-duplex mode, where data is sent and received simultaneously by both master and slave devices. The
transmission consists of 17-bit packets composed of a status/control bit and a 16-bit data word. As shown
in
, all state transitions are enabled on a rising edge of the PSTCLK clock when DSCLK is
high; that is, DSI is sampled and DSO is driven.
Figure 5-12. BDM Serial Interface Timing
DSCLK and DSI are synchronized inputs. DSCLK acts as a pseudo clock enable and is sampled on the
rising edge of the processor CLK as well as the DSI. DSO is delayed from the DSCLK-enabled CLK rising
edge (registered after a BDM state machine state change). All events in the debug module’s serial state
machine are based on the processor clock rising edge. DSCLK must also be sampled low (on a positive
edge of CLK) between each bit exchange. The MSB is transferred first. Because DSO changes state based
on an internally-recognized rising edge of DSCLK, DSDO cannot be used to indicate the start of a serial
transfer. The development system must count clock cycles in a given transfer. C1–C4 are described as
follows:
•
C1—First synchronization cycle for DSI (DSCLK is high).
•
C2—Second synchronization cycle for DSI (DSCLK is high).
•
C3—BDM state machine changes state depending upon DSI and whether the entire input data
transfer has been transmitted.
•
C4—DSO changes to next value.
NOTE
A not-ready response can be ignored except during a memory-referencing
cycle. Otherwise, the debug module can accept a new serial transfer after 32
processor clock periods.
PSTCLK
DSCLK
Next State
BDM State
Machine
DSO
DSI
Current State
Current
Next
Past
Current
C1
C2
C3
C4