8253-5 programmable interval timer pinout

DIP-24. Three 16-bit counters: time-of-day tick, DRAM refresh pacing and the speaker tone.

Three counters at port $40, each clocked at 1.19 MHz on the PC (PCLK divided by 2). Counter 0 makes IRQ0, the time-of-day tick. Counter 1 requests DRAM refresh on DMA channel 0. Counter 2, gated by PPI PB0, drives the speaker and, on the 5150, cassette data out. On the 5150 (U34) GATE0 and GATE1 are tied to +5 V and all three CLK pins share the 1.19 MHz clock. A dead timer gives the one-long-one-short beep and failing memory. PCjr (ZM23): no DMA refresh. Counter 1 helps the BIOS deserialize keyboard data, or counts timer 0 overflows during diskette I/O; port $A0 bit 5 picks its clock.

Open the interactive 8253 pinout

Pins

Pin	Name	Type	Signal
1	D7	data	Data bus bit 7.
2	D6	data	Data bus bit 6.
3	D5	data	Data bus bit 5.
4	D4	data	Data bus bit 4.
5	D3	data	Data bus bit 3.
6	D2	data	Data bus bit 2.
7	D1	data	Data bus bit 1.
8	D0	data	Data bus bit 0.
9	CLK0	clock	Counter 0 clock. PC: 1.19 MHz. PCjr: 1.1925 MHz from ZM34 (74LS112) pin 9, shared with CLK2.
10	OUT0	ctrl	Counter 0 output. PC: IRQ0, the timer interrupt.
11	GATE0	ctrl	Counter 0 gate. PCjr: tied to +5 V.
12	GND	ground	Ground.
13	OUT1	ctrl	Counter 1 output. PC: clocks the refresh-request flip-flop that drives DMA DREQ0. PCjr: not connected; the BIOS reads counter 1 to time keyboard bits.
14	GATE1	ctrl	Counter 1 gate. PCjr: tied to +5 V.
15	CLK1	clock	Counter 1 clock. PC: 1.19 MHz. PCjr: 1.1925 MHz, or timer 0 OUT0 when port $A0 bit 5 is 1.
16	GATE2	ctrl	Counter 2 gate. PC: from PPI PB0 (timer 2 gate speaker). PCjr: from 8255 PB0.
17	OUT2	ctrl	Counter 2 output. PC: speaker tone (gated with PPI PB1), read back on PPI PC5; on the 5150 also cassette data out. PCjr: speaker tone, gated with 8255 PB1 in ZM1 (74LS08); read back on 8255 PC5; also cassette data out and the IR receiver test.
18	CLK2	clock	Counter 2 clock. PC: 1.19 MHz. PCjr: 1.1925 MHz, as CLK0.
19	A0	addr	Address bit 0. With A1 selects counter 0, 1, 2 or the control word.
20	A1	addr	Address bit 1.
21	/CS	ctrl	Chip select, active LOW. PC: ports $40 to $43.
22	/RD	ctrl	Read strobe, active LOW.
23	/WR	ctrl	Write strobe, active LOW.
24	VCC	power	+5 V supply.

8253-5 programmable interval timer pinout

Pins

Fitted in

Sources