Exploring STC 8051 Microcontrollers – Coding


Serial communication is probably the most widely used communication method. It is simple to implement and can be used to communicate with a computer literally directly without needing too many extra external hardware. Serial communication also has the distance advantage over other communication interfaces. Serial communication can be implemented using UART hardware peripherals of STC micros. Many sensors, communication devices like GSM modems and RF devices, RFIDs and other external devices use this simple and time-proven interface for communicating with host micros. UART is also the backbone of other communication methods like MODBUS, IrDA, LIN, etc.

To learn more about UART visit the following link:



 #include "STC8xxx.h"
#include "BSP.h"
#include "LCD.c"
#include "lcd_print.c"
void setup(void);
void main(void)
  unsigned char msg1[10] = {"MicroArena"};
  unsigned char msg2[10] = {"SShahryiar"};
  char i = 0x00;
  char rcv_1 = 0x00;
  char rcv_4 = 0x00;
  LCD_goto(0, 0);
  LCD_putstr("TXD1: ");
  LCD_goto(10, 0);
  LCD_putstr("RXD1: ");
  LCD_goto(0, 1);
  LCD_putstr("TXD4: ");
  LCD_goto(10, 1);
  LCD_putstr("RXD4: ");
    for(i = 0; i < 10; i++)
      LCD_goto(5, 0);
      LCD_goto(5, 1);
      rcv_1 = UART1_read_buffer();  
      rcv_4 = UART4_read_buffer();
      LCD_goto(15, 0);
      LCD_goto(15, 1);
void setup(void)
  CLK_set_sys_clk(IRC_24M, 2, MCLK_SYSCLK_no_output, MCLK_out_P54);
  UART1_init(9600, \
             UART1_baud_source_TMR2, \
             UART1_timer_12T, \
  UART4_init(9600, \
             UART4_baud_source_TMR4, \
             UART4_timer_1T, \



STC8A8K64S4A12 has four hardware UART peripherals. Of these four, UART1 has some advanced features that are rarely needed. The rest are pretty much alike. Like other communication hardware peripherals, pin configuration must be selected as alternative pin arrangements are available.

RXD         TXD         Hex            Option
P0.2        P0.3        0x00           option 1
P5.2        P5.3        0x04           option 2
#define UART4_pin_option(value)        do{P_SW2 |= value;}while(0)

UART, although asynchronous, need to transmit and receive data in timed-frame formats and so when two UART devices need to communicate with each other, they must negotiate a mutually agreed baud rate or else data will not recognized. It is like tuning to the right radio station for listening the music channel that we want to listen. Thus, baud rate generation is a very crucial part of UART peripheral. For baud rate generation, UARTs can either use Timer 2 or other timer having the same number as the UART itself, for example, in the case of UART3, only Timer 2 and Timer 3 can be used as baud rate generator. I personally recommend that we keep Timer 2 reserved for other applications or UART2 and use other timers independently as to avoid conflicts in different UART hardware. Yes, I know it contradicts with my past statement of keeping Timer 2 reserved for UART applications but what else can be done when multiple UARTs are used in an application. BSP functions take care of everything internally and so we can focus on coding.

The system clock is set to 12MHz. This is very important because timers are responsible for baud rate generation and are dependent on system clock.

 CLK_set_sys_clk(IRC_24M, 2, MCLK_SYSCLK_no_output, MCLK_out_P54); 

To setup UART, we need to specify baud rate, clock source of baud rate generator, i.e., the timer to be used, its prescale factor and system clock speed in hertz. 

 UART4_init(9600, \
           UART4_baud_source_TMR4, \
           UART4_timer_1T, \

UART reading and writing is done as shown below.

rcv_4 = UART4_read_buffer();

The demo here uses two built-in UARTs – UART1 and UART4, crisscrossed among themselves. Each transmitting and receiving the other’s message. Whatever each is sending and receiving is also shown on an LCD display.


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  • Thanks for these tutorials. I’m getting back into STCmicro coding now, having left them alone for the past several years. Back then I only used the STC89C52RC (and C54RD) but this time I’m also using the more powerful STC15 and STC8 types. Your blogs provide a wealth of useful information.

  • Hello,

    You have done great job with all these tutorials. I am an electronics engineer trying to learn some new stuff. I am located in Greece , Europe and I would like to purchase the development board that you are using and download some datasheets in English if possible but I cannot find them anywhere. Could you please help me?

  • i always get excited when you release new tutorials ,you are really doing a great job i wish i could write code and develop libraries like you.

  • Well, this is very nice and thorough tutorial indeed, many thanks!
    Unfortunately I doubt there is good any reason to learn the STC platform beyond curiosity.
    The STC 8051, although pretty evolved from the original 8051 ISA, does not offer anything crucial to justify the relatively high price of these micros and development tools along with certain cumbersomeness of this ancient platform.
    They simply can not compete even with the legacy Cortex M0 in any way. I am even not aware about any affordable debugger/emulator for them.
    All in all, I would never recommend anybody to start learning/using any 8051 without some very good reason to do so.

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