WAV Player with LPC1768 and SD card

After porting the SD card low-level IO functions that my project team had written for the AVR Atmega 8-bit series about a year ago, integration with elm-chan's FAT FS was seamless. Keeping the connections suggested by Mike, the SD card posed no difficulty at all.

Playing a WAV file with a microcontroller is a "hello world" equivalent when interfacing with the SD card. For this, the motor control PWM peripheral looked ideal since I needed the main PWM peripheral for something else. An RC low pass filter of 330 ohm and 0.1uF is good enough to filter out the PWM frequency and its harmonics while leaving the audio signal intact.

Interfacing with SD card gotcha

Elm-ChaN's tech notes on interfacing with SD card is fantastic. There are, however, some things left unsaid. Today, my colleague Thejasvi and I discovered two aspects not explicit in that tutorial.

1. The way /CS signal should be handled.

Before sending a command, make /CS low. Get the response, data packets ( if any ). Now, it is essential to make /CS high before sending the next command.

2. After one command is complete. That is to say, a command is sent and a response is got (mostly R1 response) , do NOT make /CS high. The card will go out of sync and wont accept new commands properly. After the R1 response is got, send atleast one dummy byte(0xFF) and then make /CS high. Now it is ready to accept new commands. This is probably because the card is completely inactive if no data is transmitted to it as no clock signal goes to it. My conjecture is that it needs a few clocks to resynchronize after a command-response before it can accept a new command. Thus the dummy byte after the R1 response.