LED POV magic with MSP430

by filed under MSP430 MCU, Programming.
LED POV magic with MSP430
The LED POV shining in the dark.

The LED POV shining in the dark.

This simple project exploits the Persistence Of Vision (POV) of 8 LEDS connected to a port on the MSP430G2211 MCU, this is the chip I had available but the project should also work on other 14-pin devices of the same family. I’ve built this boards as a New Year’s present for my nephews.

Schematics

The scheme is pretty straightforward, 8 LEDs, each connected with its own resistor and one MCU port, a battery, a switch on the supply line and and optional programming (SBW)  connector. MSP430 LED POV Scheme

 The board

The board uses 0805 SMD parts except for the MCU which is 14-pin PDIP through-hole, I’ve made the PCB with toner-transfer technique and H2O2 + HCl etchant. Soldering this tiny parts requires some experience, a fine soldering iron tip and a good lens. Don’t forget to test every single resistor for bridges while soldering, this makes  much easier to find a problem. I connected the botton of the board with a thick thread, so that you can rotate it quickly in the air while keeping a cape in your hand.
MSP 430 LED POV PCB

MSP 430 LED POV PCB

The PCB chematics:
MSP 430 LED POV PCB scheme

MSP 430 LED POV PCB scheme

Follow the source, Luke!

Here is the code, the font file I used is available everywere on the net, for example here. 
/**
 *
 * POV LEDS
 *
 * ../compile.sh 2211_c_pov.c "-mmcu=msp430g2211"
 */

#include 

#include "font8x6.h"

volatile int msg_index = 0;
volatile int char_index = 0;
volatile int toggle = 0;
volatile int msg_length;

const char message[] = "HELLO WORLD!  ";

void setup() {1
    // halt watchdog
    WDTCTL = WDTPW + WDTHOLD;

    // make  port 1 pins outputs
    P1DIR = 0xFF;
    P1OUT = 0b00000000;


    /* The basic clock system control register 3 controls the oscillator
     * the auxilliary clock uses.  Change ACLK from external timer crystal
     * oscillator to internal very low-power oscillator (VLO) clock,
     * which runs at appoximately 12kHz.
     */
    BCSCTL3 |= LFXT1S_2;


    /* Basic clock system control register 1 controls the auxilliary clock
     * divider.  Set ACLK to use Divider 1, i.e. divided by 2^1. */
    BCSCTL1 |= DIVA_1;
    // Clock is now approximately 6KHz

    /* Set TimerA to use auxilliary clock TASSEL_1 and count up mode MC_1 */
    TACTL = TASSEL_1 + MC_1;

    /* Set counter */
    // POV: Should be around 5 - 40 ms
    TACCR0 = 4;
    

    /* Set capture/compare mode interrupts enabled, to trigger an interrupt
     * when TACCR0 is reached. */
    TACCTL0 = CCIE;

    // Message length (minus 1)
    msg_length = strlen(message) - 1;

}

void main(void)
{
    setup();
    /* Go into low power mode 3, general interrupts enabled */
    __bis_SR_register( LPM3_bits + GIE );
}


char reverseByte(char b) {
   b = (b & 0xF0) >> 4 | (b & 0x0F) << 4;
   b = (b & 0xCC) >> 2 | (b & 0x33) << 2;
   b = (b & 0xAA) >> 1 | (b & 0x55) << 1;
   return b;
}

/* Timer A interrupt service routine.  The function prototype
 * tells the compiler that this will service the Timer A0 interrupt,
 * and then the function follows.
 */
void Timer_A_isr(void) __attribute__((interrupt(TIMERA0_VECTOR)));
void Timer_A_isr(void)
{
    if(toggle){
        P1OUT = reverseByte(Font8x5[message[msg_index]][char_index]);
        // Char is 8x6, max column index is 5
        if(char_index == 5){
            // End charachter
            char_index = 0;
            if(msg_index == msg_length){
                // End message
                msg_index = 0;
            } else {
                msg_index ++; 
            }               
        } else {
           char_index++; 
        }
    } else {
        P1OUT = 0x00;
    }
    toggle ^= 1;
    /* clear interrupt flag */
    TACCTL0 &= ~CCIFG;
}