Booting from secondary SD card in Pi Zero?

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Poi
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Joined: Mon Jan 07, 2019 11:57 pm

Booting from secondary SD card in Pi Zero?

Postby Poi » Wed Jun 12, 2019 10:01 am

Would it be possible to boot from a second sd card board wired to a Pi zero through the GPIO pins?

Thinking of trying it out because I already have a couple of SD card breakout boards and I like the bigger sd cards better than the micro ones because they are easier to handle.

At the moment the only thing I am sure about is which cables to connect from the board to the Pi zero, but I am not really clear on wether it is possible to completely ignore the built in micro sd card slot or not.

Any thoughts on this?
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Ultibo
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Re: Booting from secondary SD card in Pi Zero?

Postby Ultibo » Thu Jun 13, 2019 12:01 am

Poi wrote:Would it be possible to boot from a second sd card board wired to a Pi zero through the GPIO pins?

Booting from the secondary SD card is a function of the firmware that is burned into the SoC during manufacture. According to the documentation here it is supported by the SoC but not enabled in the current BCM283x as per this quote:

The primary SD card boot mode is, as standard, set to be GPIOs 49-53. It is possible to boot from the secondary SD card on a second set of pins, i.e. to add a secondary SD card to the GPIO pins. However, we have not yet enabled this ability.

I guess that means it won't be possible to boot from the secondary SD interface unless Raspberry Pi make the required changes to the SoC firmware.

If you don't want to boot from it and just want to access the SD card on the alternate pins (GPIO22 to GPIO27) then that should be no problem at all to setup, although at present we only support the SDHCI host device and not the alternate SDHOST device so you couldn't have both at once.
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Gavinmc42
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Re: Booting from secondary SD card in Pi Zero?

Postby Gavinmc42 » Thu Jun 13, 2019 8:44 am

I have only programmed the OTP boot mode option on Pi3B's.
The Pi3B+ come with it preprogrammed, both use the BCM2837.

Does the Zero(BCM2835) have OTP boot options?
Compute module has eMMC and USB modes

https://www.raspberrypi.org/documentati ... otpbits.md
https://www.raspberrypi.org/documentati ... ootflow.md

The primary SD card boot mode is, as standard, set to be GPIOs 49-53. It is possible to boot from the secondary SD card on a second set of pins, i.e. to add a secondary SD card to the GPIO pins. However, we have not yet enabled this ability.

NAND boot and SPI boot modes do work, although they do not yet have full GPU support.

I expect somewhere there is documentation that describes booting a minimum boot loader from NAND flash or SPI flash chip.

Wonder how hard it would be to figure those out?
Probably best to try with a Compute module, less chance of bricking it?

There is probably an config.txt or comdline.txt boot option
Poi
Posts: 36
Joined: Mon Jan 07, 2019 11:57 pm

Re: Booting from secondary SD card in Pi Zero?

Postby Poi » Thu Jun 13, 2019 8:03 pm

Thanks for the answers guys. In the end I used a micro sd to sd extender to meet my needs of using a larger card a little bit further away from the Pi. Not so fancy a solution, but it works just fine.

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