PCF8574-based I2C LCD backpacks

So it seems that there's a lot of these about. A while back I picked up a bunch of cheap LCD backpacks on eBay, they look like this:

Like most of these cheap LCD backpack units, it utilises a GPIO expander chip. In this case, the chip used is a PCF8574. Most of the other I2C backpacks for Hitachi LCDs seem to utilise Microchip's MCP23008, which seems to be the little brother of the MCP23017 I've been playing with lately. So I had quite a hunt for some quick and easy code libraries to test out the backpacks with. I found one in Atlassian Bitbucket. I used version 1.2.1.

I then found this page which describes a number of the cheap eBay backpacks and clones. The third code example worked fine with the above library.

Unfortunately, after reducing the test sketch to merely the below, it was still gobbling 4768 bytes of flash. Unacceptably large, I think.

At some point in the past I'd picked up a bunch of black-on-green 16x2 LCDs for next to nothing. The total cost per LCD+backpack combo should come to easily less than $10 (AUD) each, including shipping. Pretty happy with that. They even work properly!

How I compile Stellaris projects under OSX

So I don't really know much about the TI Stellaris yet, having mainly used AVR chips to date. But I have a couple of the Stellaris Launchpads, and have managed to get the toolchain working under Mac OS X. I'm not going to repeat all the steps required for that here; you can use this page as a good starting point for that, and noting that you can save yourself a whole bunch of time/hassle by setting up MacPorts and doing this:

sudo port install arm-none-eabi-{gcc,binutils,gdb}

In my $HOME/Code/stellaris directory I have a single Makefile called Makefile.simple, and a separate subdirectory for each project. Its contents look like this:

CC=arm-none-eabi-gcc
LD=arm-none-eabi-ld
OBJCOPY=arm-none-eabi-objcopy
CPU=cortex-m4
STELLARIS_TOP=$(HOME)/build/stellaris/stellaris
CFLAGS_STELLARIS=-mthumb -mcpu=$(CPU) -mfpu=fpv4-sp-d16 -mfloat-abi=softfp -ffunction-sections -fdata-sections -MD -I$(STELLARIS_TOP)
CFLAGS=-std=c99 -Wall -Os $(CFLAGS_STELLARIS)
LDFLAGS_LEFT=-T $(PROG).ld --entry ResetISR 
LDFLAGS_RIGHT=--gc-sections

$(PROG).bin: $(PROG).o startup_gcc.o $(PROG).ld
 $(LD) $(LDFLAGS_LEFT) -o $(PROG).bin $(PROG).o startup_gcc.o $(LDFLAGS_RIGHT)

$(PROG).out: $(PROG).bin
 $(OBJCOPY) -O binary $(PROG).bin $(PROG).out

clean:
 $(RM) $(PROG).bin $(PROG).out $(PROG).[od] startup_gcc.[od]

flash: $(PROG).out
 lm4flash $(PROG).out

You can see in the above that STELLARIS_TOP is set to the location in which I have unpacked (actually git clone...) StellarisWare. This is provided to gcc so that it can find C include files. Adjust as necessary if you are using my Makefile

In each project subdirectory I have a few files:

$ ls
Makefile       blinky.c       blinky.ld      startup_gcc.c

The Makefile merely includes ../Makefile.simple and specifies the name of this project:

PROG=blinky
-include ../Makefile.simple

The other files (blinky.ld and startup_gcc.c), at least at this learning stage, are the same for every project; I just copy them over from project to project.

To build my project, I just type make:

$ make
arm-none-eabi-gcc -std=c99 -Wall -Os -mthumb -mcpu=cortex-m4 -mfpu=fpv4-sp-d16 -mfloat-abi=softfp -ffunction-sections -fdata-sections -MD -I/Users/jslee/build/stellaris/stellaris   -c -o blinky.o blinky.c
arm-none-eabi-gcc -std=c99 -Wall -Os -mthumb -mcpu=cortex-m4 -mfpu=fpv4-sp-d16 -mfloat-abi=softfp -ffunction-sections -fdata-sections -MD -I/Users/jslee/build/stellaris/stellaris   -c -o startup_gcc.o startup_gcc.c
arm-none-eabi-ld -T blinky.ld --entry ResetISR  -o blinky.bin blinky.o startup_gcc.o --gc-sections

My project compiled and linked OK, so it is ready to send to the Launchpad with lm4flash:

$ make flash
arm-none-eabi-objcopy -O binary blinky.bin blinky.out
lm4flash blinky.out
ICDI version: 9270

Please let me know if you found this useful! I also use a similar Makefile structure for building and flashing AVR and MSP430 projects.