AT89S52 (8051)

Fig. 1. AT89S52 (8051), PDIP-40. Hover or tap a signal to light up every pin that carries it.

• Port 0 is open drain with no internal pull-ups. To use it as ordinary output you need an external pull-up resistor on every bit, typically a 10k resistor network. Ports 1, 2 and 3 have internal pull-ups and do not.
• EA must be tied HIGH to run from internal flash. Tied low the chip ignores its own memory and fetches from an external ROM. Floating is why a freshly wired 8051 does nothing at all.
• RESET is ACTIVE HIGH, which is backwards from nearly every other microcontroller. The standard circuit is a 10 uF cap from VCC to the pin and a 10k resistor to ground.
• 11.0592 MHz is the traditional crystal because it divides exactly into 9600 and the other standard baud rates. 12 MHz gives round instruction timings but ugly baud rates.
• The classic 8051 takes 12 clock cycles per machine cycle, so an 11.0592 MHz part runs under a million instructions a second. Single-cycle cores like the STC and the Silicon Labs C8051 are an order of magnitude faster at the same crystal.
• The S in AT89S52 means in-system programmable over SPI, using MOSI, MISO and SCK on P1.5, P1.6 and P1.7. The older AT89C52 needs a parallel programmer.
• Ports 0 and 2 become the address and data bus if you attach external memory, which leaves you Port 1 and Port 3 for everything else.

• Use the quasi-bidirectional ports

  SDCC or Keil C51

  #include <reg52.h>
  
  /* There is no direction register. Write 1 to a bit to release it
     so it can be read, write 0 to pull it low. */
  
  P2 = 0x00;                     /* drive all of Port 2 low  */
  P2 = 0xFF;                     /* release it, weakly high  */
  
  P1 = 0xFF;                     /* release Port 1 to read it */
  if ((P1 & 0x01) == 0) {        /* P1.0 pulled to ground     */
  }
  
  sbit LED = P2 ^ 0;             /* single bit access */
  LED = 0;

  Port 0 has no internal pull-ups at all, so it cannot drive a logic high without an external resistor on every bit. Ports 1, 2 and 3 have weak ones and can.

Table 2. Pin assignments.