#395 AT89C2051 Blinky

Simplest LED blinky program in C for the AT89C2051(8051/MCS51) microprocessor, built with the SDCC open-source toolchain on macOS. Originally tested with Intel-based macOS, but now updated for Apple Silicon.

Build

Here’s a quick demo..

Notes

The main purpose of this program is simply to have something for testing the LEAP#394 AT89C2051 Programmer, and also my first trial of SDCC - Small Device C Compiler.

It is almost but not quite the simplest LED blinky program possible. It blinks an LED on P1_0, using a do-nothing loop to approximate 1Hz cycle time.

Installing SDCC

SDCC can be compiled from source or installed with a range of pre-compiled distributions available for Linux, Mac and Windows.

I previously installed SDCC using a pre-compiled distribution for macOS, specifically this version, but that was when I was still using an Intel-based machine.

I now run macOS on Apple Silicon (M3), and it seems there are no pre-compiled distributions for this platform. Can it be compiled from source? Luckily it seems the hard work has already been done - there is a brew formula that is Apple Silicon-compatible:

$ brew install sdcc
...
==> Pouring sdcc--4.5.0.arm64_sequoia.bottle.tar.gz
🍺  /opt/homebrew/Cellar/sdcc/4.5.0: 1,942 files, 146.1MB
==> Running `brew cleanup sdcc`...
...
==> Caveats
Emacs Lisp files have been installed to:
  /opt/homebrew/share/emacs/site-lisp/sdcc
$ which sdcc
/opt/homebrew/bin/sdcc

Homebrew appears to setup the environment sufficiently, but in other circumstances it may be necessary to set the SDCC_HOME environment variable so the compiler can find all its bits and pieces (like library header files).

export SDCC_HOME=./sdcc
export PATH=$PATH:${SDCC_HOME}/bin

Compiling

See the Makefile for the couple of simple instructions that are used. It builds simply like this:

$ make
sdcc -mmcs51 --code-size 2048 Blinky.c -o Blinky.ihx
packihx Blinky.ihx > Blinky.hex
packihx: read 16 lines, wrote 21: OK.

I’ve checked-in all the products of the compilation for study purposes:

File	Description
Blinky.asm	Assembler source file created by the compiler
Blinky.c	C source file
Blinky.hex	Optional packed hex file created by packihx
Blinky.ihx	The load module in Intel hex format
Blinky.lk	linker options (I think)
Blinky.lst	Assembler listing file created by the Assembler
Blinky.map	The memory map for the load module, created by the Linker
Blinky.mem	A file with a summary of the memory usage
Blinky.rel	Object file created by the assembler, input to Linkage editor
Blinky.rst	Assembler listing file updated with link edit information, created by linkage editor
Blinky.sym	Symbol listing for the sourcefile, created by the assembler

Controlling the RST line

I haven’t found the definitive specification for this yet - but I quickly learned that you can’t just ground the RST pin and have the chip run!

The AT89C2051 data sheet actually specifies a RST pull-down of 50kΩ-300kΩ, but it seems to be important to also couple to VCC with a capacitor of say 2.2µF or more. Googling for 8051 circuit schematics, I see a common combination is 8.2kΩ pull-down and 2.2µF or 10µF to VCC.

Construction

Breadboard

Schematic