Simple library for SD Cards on the Pico
At the heart of this library is ChaN's FatFs - Generic FAT Filesystem Module. It also contains a Serial Peripheral Interface (SPI) SD Card block driver for the Raspberry Pi Pico, derived from SDBlockDevice from Mbed OS 5. It is wrapped up in a complete runnable project, with a little command line interface, some self tests, and an example data logging application.
- Supports multiple SD Cards
- Supports multiple SPIs
- Supports multiple SD Cards per SPI
- Supports Real Time Clock for maintaining file and directory time stamps
- Supports Cyclic Redundancy Check (CRC)
- Plus all the neat features provided by FatFS
Using a Debug build: Writing and reading a file of 0xC0000000 (3,221,225,472) random bytes (3 GiB) on a SanDisk 32GB card with SPI baud rate 12,500,000:
- Elapsed seconds 4113.8
- Transfer rate 764.7 KiB/s
- Reading (and verifying)
- Elapsed seconds 3396.9
- Transfer rate 926.1 KiB/s
On a SanDisk Class 4 16 GB card, I have been able to push the SPI baud rate as far as 20,833,333 which increases the transfer speed proportionately (but SDIO would be faster!).
- Raspberry Pi Pico
- Something like the SparkFun microSD Transflash Breakout
- Breadboard and wires
- Raspberry Pi Pico C/C++ SDK
|MOSI||TX||16||21||DI||DI||Master Out, Slave In|
|CS0||CSn||17||22||SS or CS||CS||Slave (or Chip) Select|
|MISO||RX||19||25||DO||DO||Master In, Slave Out|
|3v3||36||3v3||3.3 volt power|
- The wiring is so simple that I didn't bother with a schematic. I just referred to the table above, wiring point-to-point from the Pin column on the Pico to the MicroSD 0 column on the Transflash.
- Card Detect is optional. Some SD card sockets have no provision for it. Even if it is provided by the hardware, you might not care about it if you intend to leave the card in all the time, so you can skip it and save an I/O pin.
- You can choose to use either or both of the Pico's SPIs.
- Wires should be kept short and direct. SPI operates at HF radio frequencies.
Pull Up Resistors
- The SPI MISO (DO on SD card, SPIx RX on Pico) is open collector (or tristate). It should be pulled up. The Pico internal gpio_pull_up is weak: around 56uA or 60kΩ. It's best to add an external pull up resistor of around 5kΩ to 3.3v. You might get away without one if you only run one SD card and don't push the SPI baud rate too high.
- The SPI Slave Select (SS), or Chip Select (CS) line enables one SPI slave of possibly multiple slaves on the bus. This is what enables the tristate buffer for Data Out (DO), among other things. It's best to pull CS up so that it doesn't float before the Pico GPIO is initialized. It is imperative to pull it up for any devices on the bus that aren't initialized. For example, if you have two SD cards on one bus but the firmware is aware of only one card (see hw_config.c); you can't let the CS float on the unused one.
- Follow instructions in Getting started with Raspberry Pi Pico to set up the development environment.
- Install source code:
git clone --recurse-submodules [email protected]:carlk3/no-OS-FatFS-SD-SPI-RPi-Pico.git no-OS-FatFS
sd_driver/hw_config.cto match hardware
pico_enable_stdio_usbin CMakeLists.txt as you prefer
cd no-OS-FatFS mkdir build cd build cmake .. make
- Program the device
- Connect a terminal. PuTTY or
tiowork OK. For example:
tio -m ODELBS /dev/ttyACM0
- Press Enter to start the CLI. You should see a prompt like:
helpcommand describes the available commands:
setrtc <DD> <MM> <YY> <hh> <mm> <ss>: Set Real Time Clock Parameters: new date (DD MM YY) new time in 24-hour format (hh mm ss) e.g.:setrtc 16 3 21 0 4 0 date: Print current date and time lliot <drive#>: !DESTRUCTIVE! Low Level I/O Driver Test e.g.: lliot 1 format [<drive#:>]: Creates an FAT/exFAT volume on the logical drive. e.g.: format 0: mount [<drive#:>]: Register the work area of the volume e.g.: mount 0: unmount <drive#:>: Unregister the work area of the volume chdrive <drive#:>: Changes the current directory of the logical drive. <path> Specifies the directory to be set as current directory. e.g.: chdrive 1: getfree [<drive#:>]: Print the free space on drive cd <path>: Changes the current directory of the logical drive. <path> Specifies the directory to be set as current directory. e.g.: cd 1:/dir1 mkdir <path>: Make a new directory. <path> Specifies the name of the directory to be created. e.g.: mkdir /dir1 ls: List directory cat <filename>: Type file contents simple: Run simple FS tests big_file_test <pathname> <size in bytes> <seed>: Writes random data to file <pathname>. <size in bytes> must be multiple of 512. e.g.: big_file_test bf 1048576 1 or: big_file_test big3G-3 0xC0000000 3 cdef: Create Disk and Example Files Expects card to be already formatted and mounted start_logger: Start Data Log Demo stop_logger: Stop Data Log Demo
- The first thing to try is lowering the SPI baud rate (see hw_config.c). This will also make it easier to use things like logic analyzers.
- Try another brand of SD card. Some handle the SPI protocol better than others. (Most consumer devices like cameras or PCs use the SDIO interface.) I have had good luck with SanDisk.
- Tracing: Most of the source files have a couple of lines near the top of the file like:
#define TRACE_PRINTF(fmt, args...) // Disable tracing //#define TRACE_PRINTF printf // Trace with printf
You can swap the commenting to enable tracing of what's happening in that file.
- Logic analyzer: for less than ten bucks, something like this Comidox 1Set USB Logic Analyzer Device Set USB Cable 24MHz 8CH 24MHz 8 Channel UART IIC SPI Debug for Arduino ARM FPGA M100 Hot and PulseView - sigrok make a nice combination for looking at SPI, as long as you don't run the baud rate too high.
There is a example data logging application in
data_log_demo.c. It can be launched from the CLI with the
start_logger command. (Stop it with the
stop_logger command.) It records the temperature as reported by the RP2040 internal Temperature Sensor once per second in files named something like
/data/2021-03-21/11.csv. Use this as a starting point for your own data logging application!
If you want to use FatFs_SPI as a library embedded in another project, use something like:
git submodule add [email protected]:carlk3/no-OS-FatFS-SD-SPI-RPi-Pico.git
git submodule add https://github.com/carlk3/no-OS-FatFS-SD-SPI-RPi-Pico.git
You will need to pick up the library in CMakeLists.txt:
add_subdirectory(no-OS-FatFS-SD-SPI-RPi-Pico/FatFs_SPI build) target_link_libraries(_my_app_ FatFs_SPI)
Appendix: Adding Additional Cards
When you're dealing with information storage, it's always nice to have redundancy. There are many possible combinations of SPIs and SD cards. One of these is putting multiple SD cards on the same SPI bus, at a cost of one (or two) additional Pico I/O pins (depending on whether or you care about Card Detect). I will illustrate that example here.
To add a second SD card on the same SPI, connect it in parallel, except that it will need a unique GPIO for the Card Select/Slave Select (CSn) and another for Card Detect (CD) (optional).
|Name||SPI0||GPIO||Pin||SPI||MicroSD 0||MicroSD 1|
|CS1||15||20||SS or CS||CS|
|CS0||17||22||SS or CS||CS||CS|
As you can see from the table above, the only new signals are CD1 and CS1. Otherwise, the new card is wired in parallel with the first card.
sd_driver/hw_config.cmust be edited to add a new instance to
static sd_card_t sd_cards
ff14a/source/ffconf.h. In particular,
#define FF_VOLUMES 2