06. 8051 Architecture · SFR · reg52.h

What you'll learn

• 1Outline the 8051 structure (CPU·ROM/RAM·ports·timer·serial·interrupts)
• 2Explain what an SFR is and where it lives (0x80–0xFF)
• 3Understand how <reg52.h> exposes SFRs by name
• 4Use whole-port writes (P1 = ...) and bit writes (sbit)
• 5Know the C51-only keywords sfr·sbit·bit

Introduction

The 8051 is simple with few registers, making it the best chip to learn "how C touches hardware". This track's 8051 examples are verified in μVision's built-in simulator (Peripherals → I/O-Ports); no real board needed.

Key concepts

1) 8051 at a glance

2) SFR — Special Function Register

SFRs control peripherals and live in internal RAM 0x80–0xFF. SFRs whose address is a multiple of 8 (0x80·0x88·0x90…) — P0–P3·TCON·SCON·IE — are bit-addressable via sbit.

3) <reg52.h> and two ways to write a port

#include <reg52.h>     /* pre-declares sfr P1 = 0x90; etc */

P1 = 0x0F;             /* (1) whole-port write: low 4 bits = 1 */

sbit LED0 = P1 ^ 0;    /* (2) sbit: name bit 0 of P1 (file scope) */
LED0 = 1;              /* only P1.0 set, others unchanged */

The ^ in `P1 ^ 0` isn't XOR — it's C51 notation for "bit 0 of P1". sfr·sbit·bit are Keil C51 extensions not in standard C, so this main.c won't compile with PC gcc (verify logic in pc_test.c).

Core example

Verify the final value after a whole-port write plus single-bit edits on PC (mimicking the SFR with a plain variable).

#include <stdio.h>
int main(void) {
    unsigned char P1 = 0x0F;                       /* whole-port write */
    P1 = (unsigned char)(P1 & ~(1u << 0));         /* LED0 = 0 */
    P1 = (unsigned char)(P1 | (1u << 7));          /* LED7 = 1 */
    printf("P1 = 0x%02X\n", P1);                   /* 0x8E */
    return 0;
}

Simulator: in Peripherals → I/O-Ports → Port 1, confirm P1 = 0x8E (1000 1110).

Common mistakes

Q. gcc says it can't find reg52.h.

A. <reg52.h>·sfr·sbit are Keil C51-only. The device main.c builds only in μVision. On PC, verify logic with pc_test.c (standard C).

Q. sbit LED = P1 ^ 0; inside a function errors out.

A. sfr/sbit declarations must be at file scope (global). Declare them above main.

Q. A large counter with int misbehaves.

A. int is 16-bit on 8051. Use unsigned char for 8-bit port values/counters and name unsigned int(16)/unsigned long(32) for larger numbers.

Summary

• The 8051 is a simple 8-bit MCU controlling peripherals via SFRs (0x80–0xFF)
• <reg52.h> provides SFRs like P1·TMOD·IE by name
• Write a port whole (P1 = 0x0F) or by bit (sbit)
• sfr·sbit·bit are C51-only — not for PC gcc (verify logic in pc_test.c)
• Use unsigned char for 8-bit values to save resources

Exercises

1. Predict the result of writing 0xAA to P1 then flipping P1.0 and P1.7 via sbit, and verify in pc_test
2. Set only the low 4 bits of P2 while preserving the high 4 bits, using a mask
3. Create an AT89C52 empty project in μVision and watch P1 in the Port 1 window