SD Pullup Requirements¶
CMD and DATA lines D0-D3 of the slave should be pulled up by 50KOhm resistor even in 1-bit mode or SPI mode. The pullups of the slave cards should be connected even if they’re not connected to the host.
The MTDI strapping pin is incompatible with DAT2 line pull-up by default when the code flash is 3.3V. See MTDI strapping pin below.
Pullup inside Official Modules¶
For Espressif official modules, different weak pullups / pulldowns are connected to CMD, and DATA pins as below. To use these modules, these pins are required to be pulled up by 50KOhm resistors, since internal weak pullups are insufficient.
|At startup||WPU||WPD||WPD||PU for 1.8v flash; WPD for 3.3v flash||WPU|
|Strapping requirement||Low to download to flash||High for 1.8v flash; Low for 3.3v flash|
- WPU: Weak pullup
- WPD: Weak pulldown
- PU: Pullup inside the module
For Wrover modules, they use 1.8v flash, and have pullup on GPIO12 inside. For Wroom-32 Series, PICO-D4 modules, they use 3.3v flash, and is weakly pulled down internally. See MTDI strapping pin below.
Pullup on Official Devkit (WroverKit)¶
For official Wrover Kit (till version 3), some of the pullups are provided on the board as the table below. For other devkits that don’t have pullups, please connect them yourselves.
|Pullup on the Kit||PU||PU||PU||PU & PD|
- PU: Pullup
- PD: Pulldown
The DAT3 pullup conflicts with JTAG pulldown in WroverKit v3 and earlier, please either:
- pull it up by resistor less than 5KOhm (2kOhm suggested) in 4-bit mode.
- pull it up or drive it high by host or VDD3.3V in 1-bit mode.
MTDI strapping pin¶
MTDI (GPIO12) is used as a bootstrapping pin to select output voltage of an internal regulator which powers the flash chip (VDD_SDIO). This pin has an internal pulldown so if left unconnected it will read low at reset (selecting default 3.3V operation). When adding a pullup to this pin for SD card operation, consider the following:
- For boards which don’t use the internal regulator (VDD_SDIO) to power the flash, GPIO12 can be pulled high.
- For boards which use 1.8V flash chip, GPIO12 needs to be pulled high at reset. This is fully compatible with SD card operation.
- On boards which use the internal regulator and a 3.3V flash chip, GPIO12 must be low at reset. This is incompatible with SD card operation. Please check the table below to see whether your modules/kits use 3.3v flash.
|Module||Flash voltage||DAT2 connections|
|PICO-D4||3.3V||Internal PD, change EFUSE and pullup or disable DAT2 line*|
|Wrover||1.8V||Internal PU, pullup suggested|
Official devkits of different types and version mount different types of modules, please refer to the table below to see whether your devkit can support SDIO slave without steps above.
|PICO Kit||PICO-D4||3.3V (see steps below)|
|WroverKit v2 and earlier|
If your board requires internal regulator with 3.3v output, to make it compatible with SD pullup, you can either:
In the case using ESP32 host only, external pullup can be omitted and an internal pullup can be enabled using a
gpio_pullup_en(GPIO_NUM_12);call. Most SD cards work fine when an internal pullup on GPIO12 line is enabled. Note that if ESP32 experiences a power-on reset while the SD card is sending data, high level on GPIO12 can be latched into the bootstrapping register, and ESP32 will enter a boot loop until external reset with correct GPIO12 level is applied.
In the case using ESP32 slave in 1-bit mode, speicfy
SDIO_SLAVE_FLAG_DAT2_DISABLEDin the slave to avoid slave detecting on DAT2 line. Note the host will not know 4-bit mode is not supported any more by the standard CCCR register. You have to tell the host use 1-bit only.
For ESP32 host or slave, another option is to burn the flash voltage selection efuses. This will permanently select 3.3V output voltage for the internal regulator, and GPIO12 will not be used as a bootstrapping pin. Then it is safe to connect a pullup resistor to GPIO12. This option is suggested for production use. NOTE this cannot be reverted once the EFUSE is burnt.
The following command can be used to program flash voltage selection efuses to 3.3V:
components/esptool_py/esptool/espefuse.py set_flash_voltage 3.3V
This command will burn the XPD_SDIO_TIEH, XPD_SDIO_FORCE, and XPD_SDIO_REG efuses. With all three burned to value 1, the internal VDD_SDIO flash voltage regulator is permanently enabled at 3.3V. See the technical reference manual for more details.
espefuse.py has a –do-not-confirm option if running from an automated flashing script.
GPIO2 Strapping pin¶
GPIO2 pin is used as a bootstrapping pin, and should be low to enter UART download mode. You may find it unable to enter the UART download mode if you correctly connect the pullup of SD on GPIO2. For WroverKit v3, there are dedicated circuits to pulldown the GPIO2 when downloading. For other boards, one way to do this is to connect GPIO0 and GPIO2 using a jumper, and then the auto-reset circuit on most development boards will pull GPIO2 low along with GPIO0, when entering download mode.
- Some boards have pulldown and/or LED on GPIO2. LED is usually ok, but pulldown will interfere with D0 signals and must be removed. Check the schematic of your development board for anything connected to GPIO2.