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
- At least one (depending on configuration) of the two Serial Peripheral Interface (SPI) controllers is used.
- For each SPI controller used, two DMA channels are claimed with
dma_claim_unused_channel
. - DMA_IRQ_0 is hooked with
irq_add_shared_handler
and enabled. - For each SPI controller used, one GPIO is needed for each of RX, TX, and SCK. Note: each SPI controller can only use a limited set of GPIOs for these functions.
- For each SD card attached to an SPI controller, a GPIO is needed for CS, and, optionally, another for CD (Card Detect).
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:
- Writing
- 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 Adafruit Micro SD SPI or SDIO Card Breakout Board or SparkFun microSD Transflash Breakout
- Breadboard and wires
- Raspberry Pi Pico C/C++ SDK
- (Optional) A couple of ~5-10kΩ resistors for pull-ups
- (Optional) A couple of ~100 pF capacitors for decoupling
SPI0 | GPIO | Pin | SPI | MicroSD 0 | Description | |
---|---|---|---|---|---|---|
MISO | RX | 16 | 21 | DO | DO | Master In, Slave Out |
CS0 | CSn | 17 | 22 | SS or CS | CS | Slave (or Chip) Select |
SCK | SCK | 18 | 24 | SCLK | CLK | SPI clock |
MOSI | TX | 19 | 25 | DI | DI | Master Out, Slave In |
CD | 22 | 29 | CD | Card Detect | ||
GND | 18,23 | GND | Ground | |||
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, if you have no requirement for it you can skip it and save a Pico 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.
- 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); you can't let the CS float on the unused one.
- Driving the SD card directly with the GPIOs is not ideal. Take a look at the CM1624 (https://www.onsemi.com/pdf/datasheet/cm1624-d.pdf). Unfortunately, it's a tiny little surface mount part -- not so easy to work with, but the schematic in the data sheet is still instructive. Besides the pull up resistors, it's probably not a bad idea to have 40 - 100 Ω series terminating resistors at the SD card end of CS, SCK, MISO, MOSI.
- It can be helpful to add a decoupling capacitor or two (e.g., 10, 100 nF) between 3.3 V and GND on the SD card.
- Note: the Adafruit Breakout Board takes care of the pull ups and decoupling caps, but the Sparkfun one doesn't.
-
I don't think the Pimoroni Pico VGA Demo Base can work with a built in RP2040 SPI controller. It looks like RP20040 SPI0 SCK needs to be on GPIO 2, 6, or 18 (pin 4, 9, or 24, respectively), but Pimoroni wired it to GPIO 5 (pin 7).
-
The SparkFun RP2040 Thing Plus works well, on SPI1. The only downside to this board is that it's difficult to access the signal lines if you want to look at them with, say, a logic analyzer or an oscilloscope.
-
For SparkFun RP2040 Thing Plus:
SPI0 GPIO Description MISO RX 12 Master In, Slave Out CS0 CSn 09 Slave (or Chip) Select SCK SCK 14 SPI clock MOSI TX 15 Master Out, Slave In CD Card Detect
-
-
Maker Pi Pico looks like it could work on SPI1. It has CS on GPIO 15, which is not a pin that the RP2040 built in SPI1 controller would drive as CS, but this driver controls CS explicitly with
gpio_put
, so it doesn't matter.
- Follow instructions in Getting started with Raspberry Pi Pico to set up the development environment.
- Install source code:
git clone --recurse-submodules git@github.com:carlk3/no-OS-FatFS-SD-SPI-RPi-Pico.git no-OS-FatFS
- Customize:
- Configure the code to match the hardware: see section Customizing for the Hardware Configuration, below.
- Customize
ff14a/source/ffconf.h
as desired - Customize
pico_enable_stdio_uart
andpico_enable_stdio_usb
in CMakeLists.txt as you prefer. (See 4.1. Serial input and output on Raspberry Pi Pico in Getting started with Raspberry Pi Pico and 2.7.1. Standard Input/Output (stdio) Support in Raspberry Pi Pico C/C++ SDK.)
- Build:
cd no-OS-FatFS/example
mkdir build
cd build
cmake ..
make
- Program the device
This library can support many different hardware configurations.
Therefore, the hardware configuration is not defined in the library1.
Instead, the application must provide it.
The configuration is defined in "objects" of type spi_t
(see sd_driver/spi.h
) and sd_card_t
(see sd_driver/sd_card.h
).
There can be one or more objects of both types.
These objects specify which pins to use for what, SPI baud rate, features like Card Detect, etc.
An instance of spi_t
describes the configuration of one SPI controller.
An instance of sd_card_t
describes the configuration of one SD card socket.
You must provide a definition for the functions declared in sd_driver/hw_config.h
:
size_t spi_get_num()
Returns the number of SPIs to use
spi_t *spi_get_by_num(size_t num)
Returns a pointer to the SPI "object" at the given (zero origin) index
size_t sd_get_num()
Returns the number of SD cards
sd_card_t *sd_get_by_num(size_t num)
Returns a pointer to the SD card "object" at the given (zero origin) index.
The definition of the hardware configuration can either be built in at build time, which I'm calling "static configuration", or supplied at run time, which I call "dynamic configuration".
In either case, the application simply provides an implementation of the functions declared in sd_driver/hw_config.h
.
- See
simple_example.dir/hw_config.c
orexample/hw_config.c
for examples of static configuration. - See
dynamic_config_example/hw_config.cpp
for an example of dynamic configuration.
After [sd_init_driver() is now2 called implicitly.]stdio_init_all();
, time_init();
, and whatever other Pico SDK initialization is required, call sd_init_driver();
to initialize the SPI block device driver.
- Now, you can start using the FatFs Application Interface. Typically,
- f_mount - Register/Unregister the work area of the volume
- f_open - Open/Create a file
- f_write - Write data to the file
- f_read - Read data from the file
- f_close - Close an open file
- f_unmount
- There is a simple example in the
simple_example
subdirectory.
- There is a simple example in the
- There is also POSIX-like API wrapper layer in
ff_stdio.h
andff_stdio.c
, written for compatibility with FreeRTOS+FAT API (mainly so that I could reuse some tests from that environment.)
-
There is a example data logging application in
data_log_demo.c
. It can be launched from theno-OS-FatFS/example
CLI with thestart_logger
command. (Stop it with thestop_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 git@github.com:carlk3/no-OS-FatFS-SD-SPI-RPi-Pico.git
or
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)
and #include "ff.h"
.
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 | SDIO | MicroSD 0 | MicroSD 1 |
---|---|---|---|---|---|---|---|
CD1 | 14 | 19 | CD | ||||
CS1 | 15 | 20 | SS or CS | DAT3 | CS | ||
MISO | RX | 16 | 21 | DO | DAT0 | DO | DO |
CS0 | 17 | 22 | SS or CS | DAT3 | CS | ||
SCK | SCK | 18 | 24 | SCLK | CLK | SCK | SCK |
MOSI | TX | 19 | 25 | DI | CMD | DI | DI |
CD0 | 22 | 29 | CD | ||||
GND | 18, 23 | GND | GND | ||||
3v3 | 36 | 3v3 | 3v3 |
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.
hw_config.c
(or equivalent) must be edited to add a new instance tostatic sd_card_t sd_cards[]
- Edit
ff14a/source/ffconf.h
. In particular,FF_VOLUMES
:
#define FF_VOLUMES 2
- Connect a terminal. PuTTY or
tio
work OK. For example:tio -m ODELBS /dev/ttyACM0
- Press Enter to start the CLI. You should see a prompt like:
>
- The
help
command 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.
- Make sure the SD card(s) are getting enough power. Try an external supply. Try adding a decoupling capacitor between Vcc and GND.
- Hint: check voltage while formatting card. It must be 2.7 to 3.6 volts.
- Hint: If you are powering a Pico with a PicoProbe, try adding a USB cable to a wall charger to the Pico under test.
- Try another brand of SD card. Some handle the SPI interface 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.
- Get yourself a protoboard and solder everything. So much more reliable than solderless breadboard!
Footnotes
-
as of Pull Request #12 Dynamic configuration (in response to Issue #11 Configurable GPIO pins), Sep 11, 2021 ↩
-
as of Pull Request #5 Bug in ff_getcwd when FF_VOLUMES < 2, Aug 13, 2021 ↩