MCS-51

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Getting started with MCS-51 development using free software: Serial output on the C8051F300 Development Board Module

This short tutorial presents a simple "Hello World" program for the C8051F300 Development Board Module, with an extra Hobby Components HCCABL0015 USB to RS232 TTL Serial Interface Cable. 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

C8051F300 Development Board Module with Demo running

The C8051F300 Development Board Module is connected to the host computer via a U-EC6 to supply power and to write the demo onto the board. For serial ouput we attach the serial interface cable (white to P04, green to P05).

Get SDCC

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 stm8flash-master.zip) 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!" Here is the C code:

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

__sfr __at(0xe2) XBR1;
__sfr __at(0xe3) XBR2;
__sfr __at(0xd9) PCA0MD;

__sfr __at(0x88) TCON;
__sfr __at(0x89) TMOD;
__sfr __at(0x8b) TL1;
__sfr __at(0x8d) TH1;

__sfr __at(0x98) SCON0;
__sfr __at(0x99) SBUF0;

#if __SDCC_REVISION >= 9624
int putchar(int c)
#else
void putchar(char c)
#endif
{
	while(!(SCON0 & 0x02));
	SCON0 &= ~0x02;
	SBUF0 = c;

#if __SDCC_REVISION >= 9624
	return(c);
#endif
}

unsigned char _sdcc_external_startup(void)
{
    PCA0MD = 0; // Disable watchdog timer

    return 0;  // perform normal initialization
}

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

	// Initialize I/O pins
	XBR1 = 0x03;				// Enable Tx and Rx
	XBR2 = 0x40;				// Enable I/O

	// Configure UART for 9600 baud, 8 data bits, 1 stop bit.
	TMOD = 0x20;
	SCON0 = 0x40;
	TH1 = 243;
	TCON |= 0x40;
	SCON0 |= 0x02;				// Tell putchar() the UART is ready to send.

	for(;;)
	{
		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" using a terminal program configured for 9600 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: