This port controls what devices will be allowed to trigger interrupts. When an interrupt is triggered, it should be cleared by resetting the corresponding bit. Otherwise, it will continuously call the interrupt code once interrupts are re-enabled with ei.
Writing to Port
Bit 0: Set 1 to enable the ON key. Set 0 acknowledge the interrupt request and/or to disable it.
Bit 1: Set 1 to enable the first hardware timer. Set 0 acknowledge the interrupt request and/or to disable it.
Bit 2: Set 1 to enable the second hardware timer. Set 0 acknowledge the interrupt request and/or to disable it.
Bit 3: Set 0 to put the calculator in low power mode if and only if the Z80 cpu enters the halt state (during the executing of a halt instruction). Set 1 to keep the calculator powered.
Bit 4: Set 1 to enable the linkport interrupt. Set 0 acknowledge the interrupt request and/or to disable it. (Does not completely disable the linkport.)
Reading from Port
Bit 0: Set if pressing the ON key will generate an interrupt.
Bit 1: Set if the first hardware timer will generate an interrupt (range: 108Hz-512Hz).
Bit 2: Set if the second hardware timer will generate an interrupt (range: 216Hz-1024Hz).
Bit 4: Set if the linkport will generate an interrupt.
For normal operation, write 0B (00001011) to this port. This will allow the first timer to generate interrupts, and the on key.
In low power mode it is known that at least the lcd driver enters standby mode and that the timer hardware has been disabled.
This example shows how to do a soft powerdown (similar to the auto powerdown) on the calculator. This may or may not wipe the LCD screen, so you should first copy its contents to a buffer like saveSScreen and then copy it back afterwards.
push af ld a, 11h ;Enable linkport, ON key. Disable timer. ;Set low power mode. out (3), a halt ;Enter low power mode (disabling various devices) ;and wait for an interrupt (either ON key or ;link activity) which will enable all hardware ;devices again. ld a, 0Bh ;Enable ON key, timer. Disable linkport. ;Keep calculator powered. out (3), a pop af