Datasheet

Chapter 5. Debug Support 5-25
Background Debug Mode (BDM)
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.
5.5.2.1 Receive Packet Format
The basic receive packet, Figure 5-16, consists of 16 data bits and 1 status bit.
Table 5-19 describes receive BDM packet fields.
16 15 0
S Data Field [15:0]
Figure 5-16. Receive BDM Packet
Table 5-19. Receive BDM Packet Field Description
Bits Name Description
16 S Status. Indicates the status of CPU-generated messages listed below. 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.
S
Data Message
0 xxxx Valid data transfer
0 0xFFFF Status OK
1 0x0000 Not ready with response; come again
1 0x0001 Error—Terminated bus cycle; data invalid
1 0xFFFF Illegal command
15–0 Data Data. Contains the message to be sent from the debug module to the development system. The
response message is always a single word, with the data field encoded as shown above.