c) Keyboard controDer
The 8255 writes strobe (key scan signals) on PA and reads key data from PB. The table shown below is the key map.
d) Signals around the 8253
The 8253 Programmable Timer generates the buzzer tone through the counter #(J and keeps the internal timer function via the counters #1 and #2.
The counter #(J is used as a square waveform generator (Mode 3). The counter #1 is used as a rate generator (Mode 2) and #2 as an interrupt upon terminal count (Mode 0).
The counter #(J counts the input pulse of 895KHz which is divided by a predetermined factor (musical note), and is then supplied to the amplifier to generate sound. The counter #1 receives an input pulse of 15.6KHz and creates a pulse on OUTl every second. The counter #2 counts those pulses and OUT2 turns to a high level 12 hours after. As OUT2 is connected to the CPU interrupt pin, it then goes into an interrupt processing routine.
8253
| ||
OUT1 |
|
|
D7 |
|
|
ClK1 |
| BlNK 05.6KHz) |
Do |
|
|
OUTO |
|
|
|
| |
|
| sp, |
ClKO |
| 895KHz |
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