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The Interval Timer
The WC34020 includes an interval timer. This timer is controlled
through 2 device registers associated with words in the high end of
memory. The first word is the device status register. The second
holds the current value of the timer. If the value of the second word
is not zero, then it is automatically decremented by 1 each time an
instruction is executed. Its current value can be inspected by
loading its value from the memory location it occupies. Its value can
be changed by storing to its location.
Only one bit of the status register is used. This is the low-order
bit (referred to as DEVINTEN in the discussion of terminal status
registers). As in a terminal status register, this bit determines
whether or not the timer will request interrupts. If this bit's value
is 1, the timer requests an interrupt whenever the timer's value is 0.
If the value of the DEVINTEN bit in the timer status register is 0, no
interrupt is requested. When an interrupt is requested, the DEVINTEN
bit is automatically cleared to 0.