AT89S52 8051 microcontroller: pinout, ports, ISP pins and a minimal circuit

A through-hole AT89S52 is marked AT89S52-24PU or AT89S52-33PU. The number is the top clock: 24 MHz on a 4.0 to 5.5 V supply, or 33 MHz on 4.5 to 5.5 V. PU is the 40-pin 0.6 inch PDIP; AU and JU are the 44-lead TQFP and PLCC, which number their pins differently.

The AT89C52 in older kits has the same pins and Timer 2 but no in-system programming, and the AT89S51 is the 4 KB version without Timer 2. Chips from other makers sold as "89S52 compatible" are outside this page: check their own datasheet, above all for how they are programmed.

The AT89S52 and AT89C52 datasheets give the same absolute maximums. Where they part company is the working supply: 4.0 to 5.5 V for the S52, 5 V ± 20% for the C52. Run either at 5 V, and treat the 15 mA DC output current as a ceiling, not a working figure: the steady-state limit in the DC table is 10 mA per pin.

Port 0 (pins 39 to 32) is open drain. It can sink eight TTL loads, but writing a 1 only lets the pin float: there is no pull-up inside unless Port 0 is acting as the external address and data bus. Used as plain I/O it needs a resistor to VCC on every pin.

Ports 1, 2 and 3 have internal pull-ups. A 1 written to the pin makes it an input; a pin pulled low from outside then sources up to 50 µA, and up to 300 µA while it swings through 2 V. The pull-ups are weak: a high output is promised only 2.4 V at 60 µA, so these pins sink current well and source almost none. P1.0 and P1.1 double as Timer 2 inputs, P1.5 to P1.7 carry the ISP signals, Port 2 gives the high address byte for external memory, and Port 3 carries RXD, TXD, INT0, INT1, T0, T1, WR and RD.

ALE (pin 30) pulses at one sixth of the crystal frequency while the chip runs. PSEN (pin 29) is the read strobe for external program memory. EA (pin 31) chooses where code comes from: tied to VCC, the internal flash; tied to ground, external memory from address 0000H.

• Learning the 8051 instruction set and its timers, serial port and interrupts on a chip that sits in a breadboard or ZIF socket.
• 5 V control boards with up to 32 I/O lines: relays through driver transistors, keypads, seven-segment and character LCD displays.
• Keeping an existing 8051 design or lab kit running, since the pinout matches the AT89C52 and AT89S51.

It is the wrong chip for anything new that needs an ADC, PWM hardware, 3.3 V logic or low sleep current. An ATmega328P does all of those in a smaller package.

A 5 V supply, an 11.0592 MHz crystal, a power-on reset, EA tied high and pull-ups on Port 0: the least the chip needs to run its own program and talk over its serial port.

The RC time constant calculator redoes step 4 for other parts, and the RC time constant lesson explains the curve.

• Drive LEDs by sinking: anode to 5 V through a resistor, cathode to the pin, lit on a 0. For eight red LEDs of about 2 V on Port 1, (5 − 2 − 0.45) V / 1.6 mA = 1.59 kΩ, so 1.8 kΩ: 1.4 mA each and 11.3 mA for all 8, inside the 15 mA port limit (Figure 5).
• The 10 mA per-pin limit does not add up across a port. Eight pins at 10 mA would be 80 mA on a port allowed 15. Anything brighter needs a transistor per pin.
• Write a 1 to a pin before reading it as an input, on every port. On Port 0 the input also needs its pull-up, or an open button reads as noise.
• For in-system programming, the programmer holds RST high and clocks SCK below one sixteenth of the crystal: 691.2 kHz at 11.0592 MHz. The 10 µF reset capacitor does not get in the way.
• Put the 100 nF decoupling capacitor across pins 40 and 20 at the chip, and keep the crystal and its two capacitors within a couple of centimetres of pins 18 and 19.

At 10 kΩ, a Port 0 pin charging only its own 10 pF has a time constant of 100 ns. Long wires add capacitance; drop to 4.7 kΩ if edges on a long cable look slow.

Both share the 40-pin layout. The AT89S51 loses memory and Timer 2; the AT89C52 loses in-system programming and speed.

1. Measure 5 V from pin 40 to pin 20, and 5 V on EA (pin 31).
2. RST (pin 9) should jump to 5 V at power-up and fall to near 0 V within a fraction of a second. Stuck high means a shorted capacitor or a missing resistor to ground.
3. Put a frequency counter or scope on ALE (pin 30). A running chip shows 1.8432 MHz, one sixth of the crystal. Nothing there means the oscillator is not running or the chip is held in reset (unless the program has turned ALE off).
4. Read the signature with the ISP programmer: an AT89S52 answers 1EH, 52H, 06H. A different answer or none at all means wiring, RST not held high, or a different chip.

Use the AT89S52-24PU at 5 V with an 11.0592 MHz crystal and 33 pF capacitors, 10 µF and 10 kΩ on RST, EA tied to VCC, a 10 kΩ network on Port 0 and 100 nF across the supply pins, and program it in circuit through P1.5 to P1.7. Sink LEDs at about 1.4 mA each, put a transistor on anything heavier, and choose a newer chip for new designs that need an ADC or PWM.

The AT89S52-24PU is widely sold by Indian component shops and in 8051 starter kits. Read the full marking: AT89S52 rather than AT89C52 if you plan to program it with an ISP programmer, and check the signature once it is in a socket. Buy the crystal as 11.0592 MHz specifically; 11 MHz and 12 MHz parts look the same. Our guide to buying electronic components in India compares ten online sellers and covers how to spot fakes.