LED Timer


LED Timer

C8051F V2.1 (F020)

LED Timer

C8051F300 Dev. B. Mod.

LED Timer

C8051F V2.1 (F120)

LED Timer
Dhrystone Whetstone




LED Timer


LED Timer






Getting started with MCS-51 development using free software: LED and timer on the STC89 DEMO BOARD

This short tutorial is about a simple LED and timer demo for a STC89 DEMO BOARD. The author used a Debian GNU/Linux system, but the tutorial should work for other Linux distributions, *BSD or other Unices.

The tools we use are

Hardware setup

STC89 DEMO BOARD with Demo running

To supply power and to write the program the board is connected via Micro-USB.


Depending on your operating system there might be an easy way to install SDCC 3.5.0 or newer using a package system or similar (e.g. apt-get install sdcc on Debian). While SDCC 3.4.0 should be sufficient for this tutorial, you might want to try a newer version in case you encounter any bugs.

SDCC binaries or a source tarball can be downloaded from its website.

Get ec2writeflash

ec2writeflash is part of the ec2-new package. The ec2-new source can be found at its GitHub location, where there is also a download link for a zip archive of the sources. To compile it, a C compiler, such as gcc, autotools and some necessary libraries need to be installed. Unzip the archive (e.g. using unzip change into the directory stm8flash-master and type autoreconf; automake --add-missing; libtoolize; autoreconf; ./configure && make. In case there are any errors, such as header files not found, check that all necessary libraries are installed.

The Demo

We present a simple Demo that increments a 2-bit LED counter once per second. This demonstrates setting up and using an accurate timer and doing basic I/O. Here is the C code:

// Source code under CC0 1.0
#include <stdbool.h>

__sfr __at(0xb0) P3;

__sfr __at(0x88) TCON;
__sfr __at(0x89) TMOD;
__sfr __at(0x8a) TL0;
__sfr __at(0x8c) TH0;

__sfr __at(0xa8) IE;

volatile unsigned long int clocktime;
volatile _Bool clockupdate;

void clockinc(void) __interrupt(1)
	TH0 = (65536 - 922) / 256;
	TL0 = (65536 - 922) % 256;
	clockupdate = true;

unsigned long int clock(void)
	unsigned long int ctmp;

		clockupdate = false;
		ctmp = clocktime;
	} while (clockupdate);

void main(void)
	// Configure timer for 11.0592 Mhz default SYSCLK
	// 1000 ticks per second
	TH0 = (65536 - 922) / 256;
	TL0 = (65536 - 922) % 256;
	TMOD = 0x01;
	IE |= 0x82;
	TCON |= 0x10; // Start timer

		P3 = ~(clock() / 1000) & 0x03;

SDCC is a freestanding, not a hosted implemenatation of C, and allows main to return void. We set up the timer to generate an interupt once per millisecond, which allows us to implement a basic clock() function. This function is used to control the blinking of the LEDs. The C8051 has a watchdog that is active on startup, and needs to be disabled. To ensure that this happens before the watchdog can reset the device, we disable the watchdog in in _sdcc_external_startup() which is executed before the initialization of global variables.

The demo can be compiled simply by invocing sdcc using sdcc -mmcs51 --std-c99 led.c assuming the C code is in led.c. The option -mmcs51 selects the target port (mcs51). An .ihx file with a name corresponding to the source file will be generated.

Put the demo onto the board

Assuming the serial cable is connected, serial.ihx will write the demo onto the board. You need to power-cycle the board after invoking stcgal. Since the LEDs are connected to the serialdata lines which are used to transfer the program onto the ┬ÁC, they will flicker for a while. After that the demo will run and count up to 3 on the yellow LEDs, then start again at 0.

More about ec2writeflash

ec2writeflash is part of ec2drv once written by Ricky White. Since ec2drv is no longer maintained, we use the version from the ec2-new fork.

More about SDCC

SDCC was initially written by Sandeep Dutta for the MCS-51, and has a relatively conservative architecture (see Sandeep Dutta, "Anatomy of a Compiler", 2000). It has been extended by various contributors and more recently, incorporated some cutting-edge technologies, in particular in register allocation (see Philipp Klaus Krause, "Optimal Register Allocation in Polynomial Time", 2013 and "Bytewise Register Allocation", 2015). However the mcs51 backend does not have all the fancy features and optimizations that some newer backends have.

SDCC is a C compiler that aims to be compliant with the C standards.

Important compiler options for MCS-51 developers include: