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GPIO2 note updated
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Updated note on GPIO2 with informations about possible pull-down resistor.

Merges espressif/esp-idf#1451
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@Pomezny @projectgus
Pomezny authored and projectgus committed Jan 2, 2018
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Expand Up @@ -37,6 +37,8 @@ With the given pinout for SPI mode, same connections between the SD card and ESP

GPIO2 pin is used as a bootstrapping pin, and should be low to enter UART download mode. 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.

### Note about GPIO12

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:
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