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The example program is included in the project archive. The example software is written using the demo TI/IAR Kickstart C compiler.

SD card access

The example firmware starts reading the SD card at the first sector. It reads one byte at a time over the SPI interface and places the byte in the PWM duty cycle register. When the end of a sector (byte 511) is reached, the next sector is immediately loaded and initialized so that the next byte is always ready when needed.

The example firmware will copy bytes from the SD card to the PWM register until it reaches the sector defined by the variable flashDisk.lastSector. At the end of this sector, the program begins again at the first sector.

The value to use here is determined by the number of sectors consumed by the audio file. The example audio file consumes 58,447 bytes. The SD card is arranged into 512 byte sectors, so the file ends at sector 115 (sector numbering starts with 0). Update this value if you are working with a custom audio file:

flashDisk.lastSector=115; //last sector (512byte block) where file is stored on flash...

Playback rate
The playback must match the sampling rate of the audio file, or the audio will sound too fast or too slow. Since I sampled audio at 8KHz, the PWM duty cycle register should be updated with a new value 8000 times each second. I appropriated the watchdog timer (WDT) from the MSP430 to sound an alarm (an interrupt) 8000 times per second. When the timer interrupts, a bit of code copies the next byte from the SD card to the PWM duty cycle register.

The timer runs on the calibrated 8 MHz internal clock. The WDT is set to trigger every 512 counts of the internal clock (8MHZ/512), or 15,625 times per second. This is about twice as fast as we need, so the interrupt routine uses a switch that updates the audio only once every other interrupt, or 7,812.5 times per second. Not exactly 8000 samples-a-second, but the internal oscillator will vary with temperature and age anyway. Using the internal crystal keeps the design simple and the part-count low.

If you're after tighter tolerances, consider using a watch crystal on the P2.6/7 pins of the MSP430 as the timer clock source.

When the MSP430 is not copying audio to the PWM, it enters a loop that continually checks if a new audio byte should be loaded into a single byte buffer. The buffer ensures that data is available when it's needed, and that audio quality isn't effected by delays in reading data from the SD card. A ton of power could be saved by entering sleep mode between interrupts, we'll look at this again in a future article.
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