Programmable Baud Rate; Self Test Diagnostics - Commodore Computers 1581 Service Manual

1581 SERVICE MANUAL
8 PROGRAMMABLE BAUD RATE GENERATOR
The 8520 contains a programmable baud rate generator which is used for fast serial transfers. Timer
A is used for the baud rate generator. In the output mode data is shifted out on SP at 1/2 the underflow
rate of Timer A. The maximum baud rate possible is phi 2 divided by 4, but the maximum usable
baud rate will be determined by line loading and the speed at which the receiver responds to the
input data. Transmission will start following a write to the Serial Data Register (provided Timer A
is running and in continuous mode). The clock derived from Timer A appears on the CNT pin. The
Data in the Serial Data Register will be loaded into the shift register then shifted out to the SP pin.
After 8 pulses on the CNT pin, a bit in the ICR (interrupt control register) is set and if desired, an
interrupt may be generated. All incoming fast bytes generate an interrupt within the Fast Serial Drive.
Bytes are shifted out; most significant bit first.
NOTE: When the 8520 is put in output mode the SP pin (data) will go low.
Baud Rate
Phi2 = 2MHz
166k
143k
100k
50k
25k
12.5k
Hi
Timer
Value
00
00
00
00
00
00
Low
Timer
Value
06
07
10
20
40
80
9 SELF TEST DIAGNOSTICS
The 1581 performs a self-test of RAM, ROM, and Controller. If an error is detected, the DOS will
blink all LED's a specific number of times. The flash code is repeated continuously.
# OF FLASHES
RESOURCE COMPONENT
1
2
3
Zero Page
ROM
100H-1FFFH
8K x 8 RAM
25256
8K x 8 RAM
In addition, the 1581 performs a Controller Test. If a failure is detected, the error channel will con
tain 76, controller error 00,00. If an extensive test of ROM is required, the 1581 provides a signature
analysis via command 'UO>T'. If a failure is detected, the LED's will blink 4 times continuously.
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