Espressif hardware products are designed for multiple use cases which may require different pull states on pins. For this reason, the pull state of particular pins on certain products will need to be adjusted to provide the pull-ups required in the SD bus.
SD pull-up requirements apply to cases where ESP32 uses the SPI controller to communicate with SD cards. When an SD card is operating in SPI mode or 1-bit SD mode, the CMD and DATA (DAT0 - DAT3) lines of the SD bus must be pulled up by 10 kOhm resistors. Slaves should also have pull-ups on all above-mentioned lines (regardless of whether these lines are connected to the host) in order to prevent SD cards from entering a wrong state.
By default, the MTDI bootstrapping pin is incompatible with the DAT2 line pull-up if the flash voltage is 3.3 V. For more information, see :ref:`mtdi_strapping_pin` below.
This section provides an overview of compatibility issues that might occur when using SDIO (secure digital input output). Since the SD bus needs to be connected to pull-ups, these issues should be resolved regardless of whether they are related to master (host) or slave (device). Each issue has links to its respective solution. A solution for a host and device may differ.
Systems on a Chip (SoCs)
^^^^^^^^^^^^^^^^^^^^^^^^
- ESP32 (except for D2WD versions, see `ESP32 datasheet <https://www.espressif.com/sites/default/files/documentation/esp32_datasheet_en.pdf>`_):
-:ref:`sd_pull-up_no_pull-ups`
-:ref:`strapping_conflicts_dat2` for models with 3.3 V flash chip
- If your host and slave device are on seperate boards, replace one of them with a board that has pull-ups. For the list of Espressif's development boards with pull-ups, go to :ref:`sdio_dev_kits`.
- Attach external pull-ups by connecting each pin which requires a pull-up to VDD via a 10 kOhm resistor.
There is a conflict between the boot strapping requirements of the ESP32 and the SDIO protocol. For details, see :ref:`mtdi_strapping_pin`.
To resolve the conflict, you have the following options:
1. (Recommended) Burn the flash voltage selection eFuses. This will permanently configure the internal regulator's output voltage to 3.3 V, and GPIO12 will not be used as a bootstrapping pin. After that, connect a pull-up resistor to GPIO12.
Burning eFuses is irreversible! The issue list above might be out of date, so please make sure that the module you are burning has a 3.3 V flash chip by checking the information on http://www.espressif.com/. If you burn the 3.3 V eFuses on a module with a 1.8 V flash chip, the module will stop functioning.
This command will burn the `XPD_SDIO_TIEH`, `XPD_SDIO_FORCE`, and `XPD_SDIO_REG` eFuses. After all the three eFuses are burned to value 1, the internal VDD_SDIO flash voltage regulator will be permanently set to 3.3 V. You will see the following log if the burning succeeds:
For more details, see the `ESP32 Technical Reference Manual <https://espressif.com/sites/default/files/documentation/esp32_technical_reference_manual_en.pdf>`_ (PDF).
- Leave the host's DAT2 floating and directly connect the slave's DAT2 to VDD.
- For a slave device, build a firmware with the option ``SDIO_SLAVE_FLAG_DAT2_DISABLED`` and re-flash your device. This option will help avoid slave detecting on the DAT2 line. Note that 4-bit SD mode will no longer be supported by the standard Card Common Control Register (CCCR); however, the host will not be aware of that. The use of 4-bit SD mode will have to be disabled on the host's side.
.._no_pull-up_on_gpio12:
No Pull-up on GPIO12
^^^^^^^^^^^^^^^^^^^^
Your module is compatible with the SDIO protocol. Just connect GPIO12 to VDD via a 10 kOhm resistor.
When the GPIO2 pin is pulled high in accordance with the SD pull-up requirements, you cannot enter Download mode because GPIO2 is a bootstrapping pin which in this case must be pulled low.
- For boards that require shorting the GPIO0 and GPIO2 pins with a jumper, put the jumper in place, and the auto-reset circuit will pull GPIO2 low along with GPIO0 before entering Download mode.
- For boards with components attached to their GPIO2 pin (such as pull-down resistors and/or LEDs), check the schematic of your development board for anything connected to GPIO2.
If the above solutions do not work for you, please determine if it is the host or slave device that has pull-ups affecting their GPIO2, then locate these pull-ups and remove them.
MTDI (GPIO12) is used as a bootstrapping pin to select the output voltage of an internal regulator (VDD_SDIO) which powers the flash chip. This pin has an internal pull-down, so, if left unconnected, it will read low at startup, which will lead to selecting the default 3.3 V operation.
All ESP32-WROVER modules, excluding ESP32-WROVER-B, use 1.8 V flash and have internal pull-ups on GPIO12. Other modules that use 3.3 V flash have no pull-ups on the GPIO12 pin, and this pin is slightly pulled down internally.
When adding a pull-up to this pin for SD card operation, consider the following:
- For boards that do not use the internal regulator (VDD_SDIO) to power flash, GPIO12 can be pulled high.
- For boards using 1.8 V flash chips, GPIO12 needs to be pulled high at reset. This is fully compatible with the SD card operation.
- On boards using the internal regulator and a 3.3 V flash chip, GPIO12 must be pulled low at reset. This is incompatible with the SD card operation. For reference information on compatibility of Espressif's boards with the SD card operation, see :ref:`compatibility_overview_espressif_hw_sdio`.
Internal Pull-ups and Strapping Requirements
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Using external resistors is always preferable. However, Espressif's products have internal weak pull-up and pull-down resistors which can be enabled and used instead of external ones. Please keep in mind that this solution cannot guarantee reliable SDIO communication.
With that said, the information about these internal pull-ups and strapping requirements can still be useful. Espressif hardware products have different weak internal pull-ups / pull-downs connected to CMD and DATA pins. The table below shows the default pull-up and pull-down states of the CMD and DATA pins.
The following abbreviations are used in the table:
-**WPU**: Weak pull-up inside the SoC
-**WPD**: Weak pull-down inside the SoC
-**PU**: Pull-up inside Espressif modules but outside the SoC
..list-table:: Default pull-up and pull-down states of the CMD and DATA pins
