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: Serial output on the C8051F120

This short tutorial is about serial output on the C8051F V2.1 with an C8051F120 presenting a simple "Hello World" program. Boards are widely available via different channels. The one used here was bought on ebay. 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

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. For serial ouput we attach a serial cable to the RS232 header.


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 repeatedly outputs "Hello World!", once per second. The C8051F120 has a calibrated 24.5 Mhz internal oscillator. However, with the default SYSCLK of 3.0625 Mhz typical baudrates cannot be achieved accurately resulting in unreliable communication. The situation is better at higher SYCLK, so we deactivate the clock divider to get a 24.5 Mhz SYSCLK. Here is the C code:

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

#include <C8051F120.h>

int putchar(int c)
	while(!(SCON0 & 0x02));
	SCON0 &= ~0x02;
	SBUF0 = c;
	return (c);

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

	return 0; // perform normal initialization

void main(void)
	unsigned long int i = 0;

	// Initialize I/O pins
	SFRPAGE = 0xf;
	XBR0 = 0x04;				// UART0 on P0.0 and P0.1
	P0MDOUT = 0x01;				// Set port P0.0 (Uart tx) to push-pull
	XBR2 = 0x40;				// Enable crossbar

	OSCICN = 0xc3;				// Run at full 24.5 Mhz - gives better accuracy for baudrate

	// Configure UART for 1200 baud, 8 data bits, 1 stop bit.
	SFRPAGE = 0x0;
	TMOD = 0x20;
	SCON0 = 0x40;
	TH1 = 203;
	TCON |= 0x40;
	SCON0 |= 0x02;				// Tell putchar() the UART is ready to send.

		printf("Hello World!\n");
		for(i = 0; i < 147456; i++); // Sleep

SDCC is a freestanding, not a hosted implemenatation of C, and allows main to return void. In SDCC up to SDCC 3.6.0 putchar() used to return void. This is not standard compliant and was changed to int in current SDCC versions. The printf() from the standard library uses putchar() for output. Since putchar() is device-specific we need to supply it. In this case we want it to output data using UART0.

The demo can be compiled simply by invocing sdcc using sdcc -mmcs51 --std-c99 serial.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 serial.ihx --run will write the demo onto the board. You can see the "Hello world" by attaching a serial cable to the DB9 connector on the RS232 board, and using a terminal program configured for 1200 baud, no parity, 8 bits, 1 stop bit and no flow control.

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: