在 SPI 模式下使用 SD 卡时，您可以在块读取过程中取消断言 CS 吗？

Question

I have a limited buffer in my microcontroller so rather than read an entire sector, I'm trying to read N bytes from a sector on an SD card, then send those N bytes to a target device (FPGA), then repeat until the whole sector is read. In order to do that I have to deassert the chip select (CS) to the SD card and assert the CS to the FPGA. When I reassert CS on the SD card I can't seem to read any more data from that sector so I'm wondering if by deasserting CS I'm terminating that transaction on the SD card. Can't seem to find CS specs to prove this though.

Answer 1

When you first assert CS, the SD card (and every other SPI device I've ever used) expects to start a new transaction. The clock where CS first asserts is used as a synchronization point, without which it would just be an endless bitstream and would require an alternate synchronization method of some kind. Specs may not explicitly say that deasserting CS terminates a transaction, but the fact that asserting it starts a new one implies that.

I think standard capacity SD (SDSC) cards will let you read blocks smaller than 512 bytes. That will limit what cards you can use, but if that's acceptable then that's an option to consider. Otherwise your best bet (without modifying the HW) is probably to just read the block over and over as many times as you need.