MC68HC908GR16 Datasheet, PDF (248/276 Pages) Motorola, Inc – Microcontrollers

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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
2-STOP BIT 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 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.
FROM HOST
READ
READ
ADDRESS ADDRESS ADDRESS ADDRESS
HIGH
HIGH
LOW
LOW
4
1
4
1
4
1
3, 2
DATA
4
ECHO
Notes:
1 = Echo delay, 2 bit times
2 = Data return delay, 2 bit times
3 = Cancel command delay, 11 bit times
4 = Wait 1 bit time before sending next byte.
RETURN
Figure 19-15. Read Transaction
MC68HC908GR16 Data Sheet, Rev. 5.0
248
Freescale Semiconductor