mc68hc908lb8 Freescale Semiconductor, Inc, mc68hc908lb8 Datasheet - Page 215

• Manufacturer Part Number: mc68hc908lb8
• Description: M68hc08 Microcontrollers Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

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.
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.
19.3.1.6 Baud Rate
The communication baud rate is controlled by the external clock frequency or internal oscillator frequency.
Table 19-1
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:
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.
Freescale Semiconductor
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READ (read memory)
WRITE (write memory)
IREAD (indexed read)
IWRITE (indexed write)
READSP (read stack pointer)
RUN (run user program)
has the external frequency required to achieve a standard baud rate of 9600 bps. The effective
START
BIT
Wait one bit time after each echo before sending the next byte.
0
BIT 0
1
2
BIT 1
MISSING STOP BIT
3
Figure 19-13. Monitor Data Format
Figure 19-14. Break Transaction
4
BIT 2
MC68HC908LB8 Data Sheet, Rev. 1
5
6
BIT 3
7
BIT 4
NOTE
BIT 5
APPROXIMATELY 2 BITS DELAY
BEFORE ZERO ECHO
BIT 6
0
1
BIT 7
2
3
STOP
BIT
4
START
NEXT
5
BIT
6
Monitor Module (MON)
7
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