Using SDXC Cards with exFAT on Raspberry Pi Pico (SPI IF)

I tested accessing exFAT-formatted microSDXC cards with the Raspberry Pi Pico.

Raspberry Pi Pico microSD access test scene

FatFs Module

Since I wanted to support not only FAT and FAT32 but also exFAT, I did not use the sample in pico-examples but instead used theFatFs Module. FatFs can be freely used, modified, and distributed at one's own risk regardless of purpose, and has abundant implementation examples, making it invaluable for DIY microcontroller users. Since I had already verified exFAT operation with the FatFs module on the Sipeed Longan Nano, I modified from that implementation. The version used wasR0.14a_p2.

Wiring Diagram / Pin Configuration

The wiring diagram is as follows.
A 10K ohm pull-up resistor on the DAT0 line is required; without it, the card will not be recognized properly.

Wiring Diagram

The connection table is also provided below.

Pico Pin #	Pico Pin Name	Function	microSD Pin #	microSD Pin Name
4	GP2	SPI0_SCK	5	CLK
5	GP3	SPI0_TX	3	CMD
6	GP4	SPI0_RX	7	DAT0
7	GP5	SPI0_CSn	2	CD/DAT3
8	GND	GND	6	VSS
36	3V3(OUT)	3.3V	4	VDD

Notes

The wiring between Raspberry Pi Pico and the microSD card slot is very sensitive, and stable operation could not be achieved with a typical breadboard microSD card module and pin wiring (especially with SDXC cards). I added functions to adjust the output pad drive strength and slew rate, which exist in the registers but are not in the Raspberry Pi Pico GPIO library, and tried various adjustments including frequency changes and inserting damping resistors, but saw little improvement. Ultimately, I confirmed stable operation by creating a direct-insertion connection module as shown in the photo below. There appears to be room for future improvement regarding SPI access timing margin.

microSD card direct-insertion module

Verified Cards

Below is information about the cards used for verification. microSD cards use FAT, microSDHC cards use FAT32, and microSDXC cards use exFAT. None of my cards failed to be recognized; all errors were due to the access timing issues mentioned above.

pico_fatfs_test Project

https://github.com/elehobica/pico_fatfs_test I have published the project. The features are as follows. The access speed measurement method was based on theTeensy 4.0 bench project. A bench.dat file will be overwritten in the card's root folder, so please back up the card used for testing.

Serial terminal output example

Below is a brief explanation of the code.

ffconf.h

FatFs functional configuration is performed inffconf.h. For this benchmark, I used a relatively broad configuration, but when using it in actual projects, you may want to consider the following parameters.

Current settings and considerations for other projects

tf_card.c / tf_card.h

The device-dependent part of the FatFs module is described entirely intf_card.c / tf_card.h.
For reference, the SPI access implementation is excerpted below. Single-byte bidirectional access, multi-byte read access, and multi-byte write access are implemented using pico-sdk hardware/spi.h functions.

/* Exchange a byte */
static
BYTE xchg_spi (
	BYTE dat	/* Data to send */
)
{
	uint8_t *buff = (uint8_t *) &dat;
	spi_write_read_blocking(spi0, buff, buff, 1);
	return (BYTE) *buff;
}


/* Receive multiple byte */
static
void rcvr_spi_multi (
	BYTE *buff,		/* Pointer to data buffer */
	UINT btr		/* Number of bytes to receive (even number) */
)
{
	uint8_t *b = (uint8_t *) buff;
	spi_read_blocking(spi0, 0xff, b, btr);
}

...

/* Transmit multiple byte */
static
void xmit_spi_multi (
	const BYTE *buff,		/* Pointer to data buffer */
	UINT btx		/* Number of bytes to transmit (even number) */
)
{
	const uint8_t *b = (const uint8_t *) buff;
	spi_write_blocking(spi0, b, btx);
}

hardware_gpio_ex

Although not used in this project, functions for setting GPIO output pad drive strength, slew rate, and input pad Schmitt trigger were implemented in thehardware_gpio_ex directory below.
By adding the following to the project's CMakeLists.txt

add_subdirectory(hardware_gpio_ex)

// ... (other settings)

target_link_libraries(${bin_name} PRIVATE
    pico_stdlib
    // ... (other libraries)
    hardware_gpio_ex
)

and including the following in the source file,
#include "hardware/gpio_ex.h"
the following three functions become available. (See commented-out sections in tf_card.c for usage examples)

void gpio_set_drive_strength(uint gpio, uint value);
void gpio_set_schmitt(uint gpio, uint value);
void gpio_set_slew_rate(uint gpio, uint value);

Access Speed Measurement Results

The access speed figures are rather unimpressive. Of course, many applications work fine at this level, so whether to use it as-is or tune for higher performance would be decided on a case-by-case basis.