General • Re: RP235X UART Boot Mode

Again:

The bootrom initialises the chip select to the following state:
• Output disabled
• Pulled high (note CSn is an active-low signal, so this deselects the external QSPI device if there is one)

Yes, but, as I would interpret that -

1. RUN goes high, RP2 reboots, Boot ROM code entered
2. Set QSPI_SS as a pulled-high input and read it
3. If not low, BOOTSEL button is not pushed, so boot from Flash
4. Set QSPI_SD1 as a pulled-low input and read it
5. If low, BOOT UART button is not pushed, so boot via USB
6. Boot via UART, configure BOOT TX and RX

So what happens at that point?

The Flash will be enabled because CS\ will be low, courtesy of QSPI_SS being a pulled-high input, but driven low by the BOOTSEL button being held.

So the question becomes whether enabled Flash could adversely interfere with BOOT TX or RX.

Assuming the PICO2-XL board uses winbond W25Q16JV like on the picture, yes it will work. All the SDX pins on the flash chip stay high impedance until you actually provide a clock to it, even with the chip select being active, and the bootrom doesn't do that.

That would appear to be the answer I was looking for.

It presumes there would be no clock supplied which would be the case with QSPI_CLK as a static output, or a pulled input, but perhaps not if it is a floating input - It seems focus should thus fall on QSPI_CLK configuration.

But, once BOOTSEL is released, the Flash CS\ is pulled high, Flash is disabled, Flash I/O gets tri-stated and everything should be fine.

a bit awkward to press/release those in that order though if they're physical buttons.

Not too bad if adjacent - RESET, BOOTSEL, UART BOOT.

Press and hold left to right to put into reset, release left to right to enter UART Boot Mode.

But it reminded me I need in-line resistors on BOOT TX and RX for cases where the interface is connected but I am not using UART Boot Mode.

Statistics: Posted by hippy — Mon Jun 30, 2025 2:14 pm

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