# Making an ATmega328P blink

Collapse OS has an AVR assembler and an AVR programmer. If you
have a SPI relay as described in the SD card recipe, then you
almost have all it takes to make an ATmega328P blink.

First, read `doc/avr.txt`. You'll see that it tells you how to
build an AVR programmer that works with your SPI relay. You
might already have such device. For example, I use the same
device as the one I connect to my Sparkfun AVR Pocket
Programmer, but I've added an on/off switch to it. I then use
a 6-pin ribbon cable to connect it to my SPI relay.

If you have a SD card connected to the same SPI relay, you'll
face a timing challenge: SD specs specifies that the minimum
SPI clock is 100kHz, but depending on your setup, you might end
up with an effective `SCK` below that. My own clock setup looks
like this:

I have a RC2014 Dual clock which allows me to have easy access
to many clock speeds, but the slowest option is 300kHz, not
slow enough. My SPI relay has a pin for input clock override,
and I built a pluggable 4040 with a switch that selects a
divisor. I plug that module in my SPI relay, then I plug that
into my RC2014 Dual clock. When doing SD card stuff, I select
the "no division" position, and when I communicate with the
AVR chip, I move the switch to increase the divisor.

Once you've done this, you can test that you can communicate
with your AVR chip by doing `160 163 LOADR` (turn off your
programmer or else it might mess up the SPI bus and prevent you
from using your SD card) and then running:

    1 asp$ aspfl@ .x 0 (spie)

(Replace `1` by your SPI device ID) If everything works fine,
you'll get the value of the low fuse of the chip.

## Building the blink binary

A blink program for the ATmega328P in Collapse OS would look
like this:

    50 LOAD ( avra ) 65 66 LOADR ( atmega328p ) H@ ORG !
    DDRB 5 SBI, PORTB 5 CBI,
    R16 TCCR0B IN, R16 0x05 ORI, TCCR0B R16 OUT,
    R1 CLR,
    L1 LBL! ( loop )
        R16 TIFR0 IN,
        R16 0 ( TOV0 ) SBRS,
            L1 ( loop ) ' RJMP LBL, ( no overflow )
        R16 0x01 LDI, TIFR0 R16 OUT,
        R1 INC,
        PORTB 5 CBI,
        R1 7 SBRS,
            PORTB 5 SBI,
        L1 ( loop ) ' RJMP LBL,

See `doc/asm.txt` for details. For now, you'll paste this into
an arbitrary unused block. Let's use `999`.

    $ cd recipes/rc2014
    $ xsel > blk/999
    $ rm blkfs
    $ make
    $ dd if=blkfs of=/dev/<your-sdcard> bs=1024

Now, with your updated SD card in your RC2014, let's assemble
this binary:

    999 LOAD
    H@ CREATE end ,
    CREATE wordcnt end ORG @ - 2 / ,
    : write 1 asp$ asperase wordcnt 0 DO
    ORG @ I 2 * + @ I aspfb! LOOP
    0 aspfp! 0 (spie) ;
    write

The first line assembles a 16 words binary beginning at `ORG @`,
then the rest of the lines are about writing these 16 words to
the AVR chip (see `doc/avr.txt` for details). After you've run
this, if everything went well, that chip if it has a LED
attached to PB5, will make that LED blink slowly.