Wednesday, January 12, 2011

ASCII Experimenter's Coding and Process, Part 1

First blog of the new year, I am excited.

All inventions start with an idea. That idea is grown over time into a design. The design is formalized and then you can start building, prototyping, discovering those things you may have missed and need to be fixed. That's where I am right now, redesigning portions of my little device. Originally I was thinking I could drive whole rows with the 74HC08's. These devices have a maximum current rating of about 25mA, which frankly isn't enough to run 4 x 20mA LEDs simultaneously. I say 4 because the 8 are split across 2 chips. Luckily I can still use them, I just need to be a bit creative.

By arranging my 2 shift registers (74HC164) in a rectangular fashion I can form a grid space of 64 states. Conveniently I have 64 LEDs I may be interested in driving, what providence! Now my 74HC08's can only drive 1 LED at a time each because the Red LEDs in my matrix run at 20 mA and I'd like to maximize this current flow for the effect of viewing the LEDs.

So if I shift in a 1 into both the row and column registers and set the OUT_EN, output enable (which ties into the 74HC08s), HIGH and then the first LED in the corner will turn on. The row register activates the current sinking transistor which will sink the current sourced from the selected 74HC08 pin.

So let's look at how to do this in code. Unfortunately I've skipped the process of designing and watching this code fail (a lot), so I will only be sharing the more decidedly interesting parts of the program in the articles to come. Most of which revolves around getting the data into the chip to begin with. Keeping in mind this is my first ever electronics project of my own design.

Right... code... I assume all I/O has been correctly setup (and I'll show this in the future) for now I'll make a little function called displayMatrix(), void return type and no arguments. Parameters are passed in global memory space (aka the stack). This function also relies on a delay function, which is a small tight loop to add a short delay of a couple of cycles, I'll include this as well:

#define LEDS P1OUT
#define OUT_EN   1
#define ROW_DATA 2
#define ROW_CLK  4
#define COL_DATA 16
#define COL_CLK  32
#define DELAY_TICKS 30

void displayMatrix(void)
        volatile unsigned int row;
        volatile unsigned int col;
        LEDS &= ~(OUT_EN | ROW_DATA | COL_DATA | ROW_CLK | COL_CLK); //Clear data and output enable
        LEDS |= ROW_DATA; //Set row data lines, start at row 1.
        LEDS ^= ROW_CLK; //Clock row data in
        LEDS ^= ROW_CLK;
        LEDS &= ~(ROW_DATA); //Disable row data
        for(row = 0; row < 8; row++) //Rows
                LEDS |= COL_DATA; //Set column data, start at column1.
                LEDS ^= COL_CLK; //Clock column data in
                LEDS ^= COL_CLK;

                LEDS &= ~(COL_DATA); //Disable column data             
                for(col = 8; col > 0; col--)//Columns
                        LEDS |= OUT_EN & (matrix[row] >> (col-1) & 0x01); //Enable output
                        LEDS &= ~(OUT_EN); //Disable output.
                        LEDS ^= COL_CLK; //Next column
                        LEDS ^= COL_CLK;
                LEDS ^= ROW_CLK; //Next row
                LEDS ^= ROW_CLK;
void delay(void)
        volatile unsigned int k;
        k = DELAY_TICKS;

This does have some requirements, for instance due to the nature of the >> and & logic I use, it requires the OUT_EN (P1.0) to be the LSB. All other pins I believe are safe from this restriction and may be moved about at will, as long as you remember to setup the bits in the #define section correctly. Of interesting note, using the TI MSP430- Launchpad I originally used P1.3 for the COL_CLK, this led to a drastically unreliable display.

As it turns out, there is enough noise on this pin when using the launchpad to trigger the 74HC164 clock an extra time or more when transitioning! This noise is caused by a switch attached to that same pin [image to come]. Rather than adding hardware to fix this, I merely avoided it's use, though I do plan on using it with 74HC151 in the future on that pin, but it'll be as an input instead of an output.

This code turns on 1 pixel at a time and splits the entire on time across all 64 pixels, displaying or not. This gives it a uniform brightness which I believe is desirable. My question to everyone else out there.
Can you see a reasonable way to turn 2 LEDs on at the same time (only 1 per 74HC08)?
I have some ideas on how to do this too and I'm willing to spare another pin for a secondary OUT_EN2. Hopefully I'll get some updated schematics up soon too, to assist anyone interested in my design stuff! Thanks :-D

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