MC68HC908LB8_05 Datasheet, PDF (215/234 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

Monitor Module (MON)
19.3.1.4 Data Format
Communication with the monitor module is in standard non-return-to-zero (NRZ) mark/space data format.
Transmit and receive baud rates must be identical.
START
BIT
BIT 0
BIT 1 BIT 2 BIT 3 BIT 4 BIT 5
NEXT
BIT 6
BIT 7
START
STOP BIT
BIT
Figure 19-13. Monitor Data Format
19.3.1.5 Break Signal
A start bit (0) followed by nine 0 bits is a break signal. When the monitor receives a break signal, it drives
the PTA0 pin high for the duration of two bits and then echoes back the break signal.
MISSING STOP BIT
APPROXIMATELY 2 BITS DELAY
BEFORE ZERO ECHO
01234567
01234567
Figure 19-14. Break Transaction
19.3.1.6 Baud Rate
The communication baud rate is controlled by the external clock frequency or internal oscillator frequency.
Table 19-1 has the external frequency required to achieve a standard baud rate of 9600 bps. The effective
baud rate is the bus frequency divided by 256 for the external oscillator and divided by 417 for the internal
oscillator. If a crystal is used as the source, be aware of the upper frequency limit that the MCU can
operate.
19.3.1.7 Commands
The monitor module firmware uses these commands:
• READ (read memory)
• WRITE (write memory)
• IREAD (indexed read)
• IWRITE (indexed write)
• READSP (read stack pointer)
• RUN (run user program)
The monitor module firmware echoes each received byte back to the PTA0 pin for error checking. An
11-bit delay at the end of each command allows the host to send a break character to cancel the
command. A delay of two bit times occurs before each echo and before READ, IREAD, or READSP data
is returned. The data returned by a read command appears after the echo of the last byte of the command.
NOTE
Wait one bit time after each echo before sending the next byte.
MC68HC908LB8 Data Sheet, Rev. 1
Freescale Semiconductor
215