Datasheet

Development Support
MC68HC908GR16 Data Sheet, Rev. 5.0
248 Freescale Semiconductor
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.
Figure 19-14. Break Transaction
19.3.1.6 Baud Rate
The communication baud rate is controlled by the crystal frequency or external clock and the state of the
PTB4 pin (when IRQ
is set to V
TST
) upon entry into monitor mode. If monitor mode was entered with V
DD
on IRQ
and the reset vector blank, then the baud rate is independent of PTB4.
Table 19-1 also lists external frequencies required to achieve a standard baud rate of 9600 bps. The
effective baud rate is the bus frequency divided by 256. If using a crystal as the clock source, be aware
of the upper frequency limit that the internal clock module can handle. See 20.7 5.0-Volt Control Timing
or 20.8 3.3-Volt Control Timing for this limit.
19.3.1.7 Commands
The monitor ROM 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 ROM 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.
Figure 19-15. Read Transaction
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
MISSING STOP BIT
2-STOP BIT DELAY BEFORE ZERO ECHO
READREAD
ECHO
FROM HOST
ADDRESS
HIGH
ADDRESS
HIGH
ADDRESS
LOW
ADDRESS
LOW
DATA
RETURN
13, 2114
4
Notes:
2 = Data return delay, 2 bit times
44
1 = Echo delay, 2 bit times
3 = Cancel command delay, 11 bit times
4 = Wait 1 bit time before sending next byte.