LED Timer


LED Timer

C8051F V2.1 (F020)

LED Timer

C8051F300 Dev. B. Mod.

LED Timer

C8051F V2.1 (F120)

LED Timer




LED Timer


LED Timer






Getting started with MCS-51 development using free software: LED and timer on the C8051F120

The C8051F V2.1 is widely available via different channels. The one used here was bought on ebay. It took some effort to get this one (the first one had an C8051F020 instead of the C8051F120 advertized, when I bought another one from a different seller, the first shipment got lost in the mail). This short tutorial is about a simple LED and timer demo for a C8051F V2.1. I 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

C8051F V2.1 board with Demo running

The C8051F V2.1 board is connected to the host computer via a U-EC6 to write the demo onto the board, while power is supplied via an extra cable.


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 blinks one LED, once per second. This demonstrates setting up and using a timer and doing basic I/O. It is very similar to the one for the C8051F020 variant of the board. Two notable differences are the frequency of the (calibrated) internal oscillator and the paged special function registers, which requires some extra setup. Here is the C code:

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

__sfr __at(0x84) SFRPAGE;

__sfr __at(0xb0) P3;
__sfr __at(0xa7) P3MDOUT;
__sfr __at(0xe3) XBR2;
__sfr __at(0xff) WDTCN;

__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 - 255) / 256;
	TL0 = (65536 - 255) % 256;
	clockupdate = true;

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

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

unsigned char _sdcc_external_startup(void)
	// Disable watchdog timer
	WDTCN = 0xde;
	WDTCN = 0xad;

	return 0; // perform normal initialization

void main(void)
	// Enable port output
	SFRPAGE = 0xf;
	XBR2 = 0x40;
	P3MDOUT = 0x08;

	// Configure timer for 3.0625 Mhz default SYSCLK
	// 1000 ticks per second
	SFRPAGE = 0x0;
	TH0 = (65536 - 255) / 256;
	TL0 = (65536 - 255) % 256;
	TMOD = 0x01;
	IE |= 0x82;
	TCON |= 0x10; // Start timer

		P3 = ((clock() / 1000) & 0x01) << 3;

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 board is connected to a U_EC6 attached via USB, ec2writeflash --port USB --hex led.ihx --run will write the demo onto the board. It will run and count up to 3 on the red 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: