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Merged
merged 35 commits into from Jul 3, 2019
Merged

Document GPIO boot mode on its own page #1131

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Document GPIO boot mode it its own page
Apr 30, 2019
3a7e16e
add content to new page
May 1, 2019
589ae4c
explain GPIO boot mode disables all boot modes by default
May 1, 2019
83ec883
add all docs from bootflow.md
May 1, 2019
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address review comments
May 1, 2019
933626e
Update gpio.md
May 1, 2019
6c7366b
SD0 = Broadcom, SD1 = Arasan with SDIO
May 2, 2019
5502987
fix wording
May 2, 2019
ed9fe20
SD0 also does MMC, remove duplicate "permanently"
May 2, 2019
6d55871
fix wording
May 2, 2019
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expand on boot order explanation
May 2, 2019
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fix typo
May 2, 2019
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Update gpio.md
May 2, 2019
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Update gpio.md
May 2, 2019
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add link to config.txt docs page
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fix relative links
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fix relative links (take 2)
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Update gpio.md
May 3, 2019
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SPI and NAND - no Linux support at present
May 3, 2019
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SPI and NAND - no Linux support at present (take 2)
May 3, 2019
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fix typo
May 3, 2019
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GPIO bank selection - revert to base of 1
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fix typo
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fix typo
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revert GPIO bank selection to base 1 (take 2)
May 3, 2019
f7ebd20
Cater for CM Lite - SD0 availability at boot to be confirmed
May 7, 2019
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Add details of SD0 on CM Lite per lurch
May 7, 2019
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SD -> microSD
May 7, 2019
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Update per Gordon's comments
May 10, 2019
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Add 3A+ and CM3 and 3+
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Update gpio.md
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Make sure all Pi models are covered by tables of GPIOs
Jun 17, 2019
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GPIO lines -> GPIO pins, since they are not in the same order
Jun 17, 2019
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Jun 17, 2019
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27 changes: 27 additions & 0 deletions hardware/raspberrypi/bootmodes/gpio.md
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# GPIO boot mode

The Raspberry Pi can be configured to allow the boot mode to be selected at power on using hardware attached to the GPIO connector: this is GPIO boot mode. This is done by setting bits in the OTP memory of the SoC. Once the bits are set, they permanently allocates 5 GPIOs to allow this selection to be made. Once the OTP bits are set they cannot be unset so you should think carefully about enabling this, since those 5 GPIO lines will then be permanently unavailable for any other use.

To enable GPIO boot mode, add the following line to the config.txt file:

```
program_gpio_bootmode=n
```

Where n is the bank of GPIOs which you wish to use. Then reboot the Pi once to program the OTP with this setting. Bank 1 is GPIOs 22-26, bank 2 is GPIOs 39-43. Unless you have a Compute Module, you must use bank 1: the GPIOs in bank 2 are only available on the Compute Module. Because of the way the OTP bits are arranged, if you first program GPIO boot mode for bank 1, you then have the option of selecting bank 2 later. The reverse is not true: once bank 2 has been selected for GPIO boot mode, you cannot select bank 1.

Once GPIO boot mode is enabled, the Raspberry Pi will no longer boot. You must pull up at least one boot mode GPIO pin in order for the Pi to boot.

Once enabled, the GPIOs are polled in numerical order as follows:

|Bank 1|Bank2|boot type|
|:----:|:---:|:-------:|
|22 |39 |SD1 |
|23 |40 |SD2 |
|24 |41 |NAND |
|25 |42 |SPI |
|26 |43 |USB |

SD1 is the <xxx> SD card interface. SD2 is the <yyy> SD card interface. USB enables both USB device mode and USB host mode boot.

The default pull resistance on the GPIO lines is 50K ohm, as documented on page 102 of the [BCM2835 ARM peripherals datasheet](hardware/raspberrypi/bcm2835/BCM2835-ARM-Peripherals.pdf). A pull resistance of 5K ohm is recommended to pull a GPIO line up or down: this will allow the GPIO to function but not consume too much power.