MOTOROLA
MC68349 USER’S MANUAL
5- 59
5.7.2.6 RETURNING FROM BDM. BDM is terminated when a resume execution (GO) or
call user code (CALL) command is received. Both GO and CALL flush the instruction
pipeline and prefetch instructions from the location pointed to by the RPC.
The return PC and the memory space referred to by the SR SUPV bit reflect any changes
made during BDM. FREEZE is negated prior to initiating the first prefetch. Upon negation
of FREEZE, the serial subsystem is disabled, and the signals revert to IPIPE and IFETCH
functionality.
5.7.2.7 SERIAL INTERFACE. Communication with the CPU32+ during BDM occurs via a
dedicated serial interface, which shares pins with other development features. The BKPT
signal becomes the DSCLK; DSI is received on IFETCH , and DSO is transmitted on
IPIPE0 .
The serial interface uses a full-duplex synchronous protocol similar to the serial peripheral
interface (SPI) protocol. The development system serves as the master of the serial link
since it is responsible for the generation of DSCLK. If DSCLK is derived from the CPU32+
system clock, development system serial logic is unhindered by the operating frequency of
the target processor. Operable frequency range of the serial clock is from DC to one-half
the processor system clock frequency.
The serial interface operates in full-duplex mode—i.e., data is transmitted and received
simultaneously by both master and slave devices. In general, data transitions occur on the
falling edge of DSCLK and are stable by the following rising edge of DSCLK. Data is
transmitted most significant bit (MSB) first and is latched on the rising edge of DSCLK.
The serial data word is 17 bits wide—16 data bits and a status/control (S/C) bit.
16
15
0
S/C
DATA FIELD
Bit 16 indicates the status of CPU-generated messages as listed in Table 5-10.
Table 5-10. CPU Generated Message Encoding
Encoding
Data
Message Type
0
xxxx
Valid Data Transfer
0
FFFF
Command Complete; Status OK
1
0000
Not Ready with Response; Come Again
1
0001
BERR Terminated Bus Cycle; Data Invalid
1
FFFF
Illegal Command
Command and data transfers initiated by the development system should clear bit 16. The
current implementation ignores this bit; however, Motorola reserves the right to use this bit
for future enhancements.