Previous: 1. Reading and Writing Bytes

2. Sending Commands

Source code: 2_sendcommand.s

This sample uses an encapsulated sd_sendcommand subroutine to send the CMD0 and CMD8 commands to the card in sequence.

Key changes:

• New subroutine sd_sendcommand for sending arbitrary commands to the card
• zp_sd_cmd_address is the address of a two-byte pointer in zero page, used to tell sd_sendcommand which data to send
• CMD0 and CMD8 commands and arguments are stored in data blocks at cmd0_bytes and cmd8_bytes

Expected result

The code will cycle through each command, sending it and waiting for a response. Before each command it clears the screen and prints the command code byte, which is $40 plus the command number. Then it prints the response, which should be 01 in both cases. There are a lot of delays to make it easier to see what's going on.

As before, if all goes well it will print a final Y, while if it detected a problem at any stage, it will print X and halt.

Troubleshooting

Assuming the previous example worked, the basic communication should work fine here too. There's only one real thing to watch out for:

c4805X

This means that CMD8 was an invalid command, and could indicate that the SD card is too old. This tutorial only supports SDHC cards. Nonetheless, you could look at Older Cards and try to adapt the code.

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Next: 3. Complete Initialization Sequence