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Figure 1. Pin Functions

Programming the CTC is straightforward: each channel is
programmed with two bytes: a third is necessary when interrupts are
enabled. Once started, the CTC counts down, automatically reloads
its time constant, and resumes counting. Software timing loops are
completely eliminated. Interrupt processing is simplified because only
one vector need be specified: the CTC internally generates a unique
vector for each channel.
The Z80 CTC requires a 5% V power supply and the standard
Z80 single-phase system clock. It is packaged in 28-pin DIPs, a
44-pin plastic chip carrier, and a 44-pin Quad Flat Pack. (Figures 2a,
2b, and 2c). Note that the QFP package is only available for CMOS
versions.

3) Pin configuration

Figure 2c. 44-pin Quad Flat Pack Pin Assignments

4) Functional description

The Z80 CTC has four independent counter/timer channels. Each
channel is individually programmed with two words: a control word
and a time-constant word. The control word selects the operating
mode (counter or timer), enables or disables the channel interrupt,
and selects certain other operating parameters. If the timing mode is
selected, the control word also sets a prescaler, which divides the
system clock by either 16 or 256. The time-constant word is a value
from 1 to 256.

During operation, the individual counter channel counts down from
the preset time constant value. In counter mode operation the counter
decrements on each of the CLK/TRG input pulses until zero count is
reached. Each decrement is synchronized by the system clock. For
counts greater than 256, more than one counter can be cascaded. At
zero count, the down-counter is automatically reset with the time
constant value.
The timer mode determines time intervals as small as 2

s (8 MHz), 3

s (6 MHz), or 4

s (4 MHz) without additional logic or software timing

loops. Time intervals are generated by dividing the system clock with
a prescaler that decrements a preset down-counter.
Thus, the time interval is an integral multiple of the clock period, the
prescaler value (16 or 256), and the time constant that is preset in the
down-counter. A timer is triggered automatically when its time con-
stant value is programmed, or by an external CLK/TRG input.
Three channels have two outputs that occur at zero count.
The first output is a zero-count/timeout pulse at the ZC/TO output.
The fourth channel (Channel 3) does not have a ZC/TO output; inter-
rupt request is the only output available from Channel 3.
The second output is Interrupt Request (INT), which occurs if the
channel has its interrupt enabled during programming. When the Z80
CPU acknowledges Interrupt Request, the Z80 CTC places an inter-
rupt vector on the data bus.
The four channels of the Z80 CTC are fully prioritized and fit into four
configuous slots in a standard Z80 daisy-chain interrupt structure.
Channel 0 is the highest priority and Channel 3 the lowest. Interrupts
can be individually enabled (or disabled) for each of the four chan-
nels.