84PCE:Ports:0000
Contents
Synopsis
Port Number: 0000
Function: Controls interrupt speed and power
Bits [1:0]: OS Timer Rate
This controls the rate of interrupt source 4, which seems to be some kind of timer. Changing this changes the cursor blink rate. It's probably some kind of interrupt speed. 11b is the slowest, 00b is the fastest. If this is anything like the timers on the Z80 models, its source is the 32768 Hz quartz crystal. A frequency table is below, along with probable divisors if this really is from the 32768 Hz crystal.
| Value | Rate (Hz) | Probable Divisor |
| 00 | ~442.81 | 32768/74 |
| 01 | ~212.78 | 32768/154 |
| 02 | ~150.31 | 32768/218 |
| 03 | ~104.36 | 32768/314 |
Bits [3:2]
Writes have no apparent effect, value does not latch.
Bit [4]
Set this bit to crash. The bootcode uses this to crash when catching a non-maskable interrupt.
Bit [5]
Writes have no apparent effect, value does not latch.
Bit [6]
Set this bit to turn calc and screen off until next ON key. Value does not latch.
Bit [7]
Latches value written, no apparent effect. Used when reading battery status.
