The Z80 CTC (counter timer circuit) is a support IC for Z80-based systems that provides four independent timer/counter channels, each able to generate interrupts, timed outputs, and system time-base functions. A device as the Zilog Z0843006 class belongs to the Z80 CTC family and is commonly used to create periodic ticks, measure external event rates, generate baud-rate clocks, and synchronize peripherals without burdening the CPU with continuous polling.

Functions

A CTC moves timing and counting tasks from software into dedicated hardware, enabling:

• Periodic interrupts (system tick, scheduler, timeouts).

• External event counting (sensors, encoders, production pulses).

• Square-wave / timing output generation for other logic blocks.

• Baud-rate clock generation or service clocks for serial interfaces.

• Deterministic synchronization with external triggers, reducing software jitter.

Functional architecture: four independent channels

The CTC integrates four channels (often numbered 0–3). Each channel provides:

• A CLK/TRG input that can act as an external clock (counter mode) or as a trigger/clock depending on configuration.

• A time constant register that defines when the terminal count event occurs.

• A prescaler path for deriving timer rates from the system clock.

• A ZC/TO output (zero count / timeout) that can produce a pulse or a square wave depending on the programmed mode.

• Per-channel interrupt capability integrated into the Z80-style daisy chain priority scheme.

Operating modes 

Per channel, you typically choose:

• Timer mode: counts system-derived ticks (via prescaler) down to zero and signals an event.

• Counter mode: counts external pulses on CLK/TRG, useful for frequency and event counting.

• Triggered operation: start counting on a defined trigger edge for “on-demand” timing windows.

• Waveform/timeout output: ZC/TO can be used as an event strobe or a periodic waveform source.

Prescaler and time constant (how the period is formed)

The usual sequence is:

• The CPU writes a control word (mode, prescaler, interrupt enable, output behavior).

• The CPU writes a time constant (N).

• The channel counts at a rate determined by clock source and prescaler.

• On terminal count, the CTC can update ZC/TO, request an interrupt, and repeat depending on mode.

CPU interface (Z80 bus and addressing)

At the bus level, a Z80 CTC connects as an I/O peripheral using:

• DATA0–DATA7

• /IORQ, /RD, /WR for I/O cycles

• /CE from address decoding

• Channel select lines (commonly A0/A1 or equivalents) to address one of the four channels

• Interrupt lines IEI/IEO for daisy chain priority and /INT to the CPU

The CPU programs control and constants, then relies on interrupts/status per the board’s I/O mapping.

Sketch of the most important connections

                 ┌───────────────────────────┐
                 │            Z80 CPU         │
                 │ DATA[0..7] /IORQ /RD /WR   │
                 │ A0..A1  /INT  /M1          │
                 └──────────────┬────────────┘
                                │ (bus + decode /CE)
                                ▼
                       ┌──────────────────┐
                       │      Z80 CTC     │  (Z0843006-type)
                       │ 4× timer/counter │
                       │ IEI/IEO daisy    │
                       └───┬────┬────┬────┘
                           │    │    │
      CH0 CLK/TRG ◄────────┘    │    └────────► CH3 ZC/TO (clock/strobe)
      CH0 ZC/TO  ───────────────┘
      CH1 CLK/TRG ◄──────────────────────────► external events/clock
      CH1 ZC/TO  ───────────────────────────► peripherals / logic
      CH2 CLK/TRG ◄──────────────────────────► sensors / encoder / clock
      CH2 ZC/TO  ───────────────────────────► tick / baud / trigger
      CH3 CLK/TRG ◄──────────────────────────► external trigger / chaining

Typical use cases

• A periodic system tick for timeouts and scheduling.

• Baud-rate clock generation for a UART or serial timing.

• Frequency measurement by counting pulses within a timed window.

• Event counting (encoder, sensors, production pulses).

• Timing chains using one channel’s ZC/TO to drive another channel’s CLK/TRG.

Table 1 – Identification data and specifications

Table 2 – Channel behavior and operational concepts (English)