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 needs to be adjusted to provide the pull-ups required in the SD bus.
SD pull-up requirements apply to cases where {IDF_TARGET_NAME} uses the SPI or SDMMC 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. SD cards and SDIO devices should also have pull-ups on all above-mentioned lines (regardless of whether these lines are connected to the host) in order to prevent them 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.
{IDF_TARGET_NAME} SDMMC host controller allows using any of GPIOs for any of SD interface signals. However, it is recommended to avoid using strapping GPIOs, GPIOs with internal weak pull-downs and GPIOs commonly used for other purposes to prevent conflicts:
- If your host and slave device are on separate 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`.
1. (Recommended) Burn the flash voltage selection eFuses. This permanently configures 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 The issue list above might be out of date, so please make sure that the module you are burning has a 3.3 V flash chip by checking the information on https://www.espressif.com/.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 burns 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 is permanently set to 3.3 V. You will see the following log if the burning succeeds:
- For a slave device, build a firmware with the option ``SDIO_SLAVE_FLAG_DAT2_DISABLED`` and re-flash your device. This option helps avoid slave detecting on the DAT2 line. Note that 4-bit SD mode is no longer supported by the standard Card Common Control Register (CCCR); however, the host is not aware of that. The use of 4-bit SD mode has to be disabled on the host's side.
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 pulls 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 level at startup, which leads 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
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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.
