Slow block read with SPI fatfs

[frnt]

Hello,

I measured the performance of uSD card connected through the SPI on Read/Write operation.

The weird thing is that the Block read speed is 3x to 4x slower than the write speed.  I'm writing/read 4kB of data and the SD card is formatted with 4kB allocation unit.

The SPI transaction of the 4k write is about 5-7ms, when considering the SPI transaction of the block read it takes about 20ms.

Does someone have an idea of what the issues can be?

[FBL]

Hello @frnt 

Which stm32 product are you using?

[frnt]

Hello @FBL,

STM32WLEx SPI bus is 16MHz

[ONadr.1]

I would look directly at the SPI bus with a logic analyzer, where the slowdown occurs.

[frnt]

Are there some important aspects to be considered in the block read except for the CMD18 command and CMD12 for stopping? I do not have a logic analyzer, I could have a look with the oscilloscope at one block transfer.

[ONadr.1]

Try to Saleale USB analyzer. Is cheap and for this work is sufficient.

[ONadr.1]

Sorry, Saleae.

[frnt]

Ok, I'll have a look at the Saleae.

Could it be that fatfs does not use DMA for receiving data and using blocking calls?

[ONadr.1]

I think reading a 4k block in 20ms is way too slow, even for a blocking function. I suggested using the analyzer to find out how the delay is spread and what could be causing it.

[frnt]

@ONadr.1 I'll wait for the purchase. In the meantime, I have tried other micro-SD cards, but same slow speed in block read. 

I'm sharing the block read code, but to me it seems correct:

---

static bool SD_RxDataBlock(BYTE *buff, UINT len)
{
	uint8_t token;
	Timer1 = 200;
	/* loop until receive a response or timeout */
	do {
		token = SPI_RxByte();
	} while((token == 0xFF) && Timer1);

	/* invalid response */
	if(token != 0xFE) return FALSE;

	/* receive */
	do {
		SPI_RxBytePtr(buff++);
	} while(--len);

	/* discard CRC */
	SPI_RxByte();
	SPI_RxByte();
	return TRUE;
}​