Z8F6421PM020SC Datasheet, PDF (216/299 Pages) Zilog, Inc. – High Performance 8-Bit Microcontrollers Z8 Encore-R 64K Series

Z8 Encore!^® 64K Series
Product Specification
196
• Asserting the RESET pin Low to initiate a Reset.
• Driving the DBG pin Low while the device is in STOP mode initiates a System Reset.
OCD Data Format
The OCD interface uses the asynchronous data format defined for RS-232. Each character
is transmitted as 1 Start bit, 8 data bits (least-significant bit first), and 1 Stop bit
(Figure 39).
START
D0
D1
D2
D3
D4
D5
D6
D7 STOP
Figure 39. OCD Data Format
OCD Auto-Baud Detector/Generator
To run over a range of baud rates (bits per second) with various system clock frequencies,
the On-Chip Debugger has an Auto-Baud Detector/Generator. After a reset, the OCD is
idle until it receives data. The OCD requires that the first character sent from the host is
the character 80H. The character 80H has eight continuous bits Low (one Start bit plus 7
data bits). The Auto-Baud Detector measures this period and sets the OCD Baud Rate
Generator accordingly.
The Auto-Baud Detector/Generator is clocked by the system clock. The minimum baud
rate is the system clock frequency divided by 512. For optimal operation, the maximum
recommended baud rate is the system clock frequency divided by 8. The theoretical maxi-
mum baud rate is the system clock frequency divided by 4. This theoretical maximum is
possible for low noise designs with clean signals. Table 99 lists minimum and recom-
mended maximum baud rates for sample crystal frequencies.
Table 99. OCD Baud-Rate Limits
System Clock Frequency Recommended Maximum Baud Rate
(MHz)
(kbits/s)
20.0
2500
1.0
125.0
0.032768 (32KHz)
4.096
Minimum Baud Rate
(kbits/s)
39.1
1.96
0.064
If the OCD receives a Serial Break (nine or more continuous bits Low) the Auto-Baud
Detector/Generator resets. The Auto-Baud Detector/Generator can then be reconfigured
by sending 80H.
PS019915-1005
On-Chip Debugger