Incorrect PICO_FLASH_SPI_CLKDIV for WaveShare RP2040-Zero

[ArkBrj]

src\boards\include\boards\waveshare_rp2040_zero.h sets PICO_FLASH_SPI_CLKDIV to 2.
However, according to the board schematics (https://www.waveshare.com/w/upload/4/4c/RP2040_Zero.pdf), this board uses Winbond W25Q16JVUXIQ chip. According to the chip's datasheet (https://www.winbond.com/resource-files/w25q16jv%20spi%20revh%2004082019%20plus.pdf), its max Clock frequency for Read Data instruction (03h) is 50 MHz (Section 10.6 AC Electrical Characteristics). PICO_FLASH_SPI_CLKDIV of 2 sets the clock to around 60 MHz exceeding the spec.
In my experiments with various binaries I see that the code always works right after flashing it, but does not work after restarting the board. This includes the officially released binary for picoprobe.
For home-built projects Debug binaries sometimes work even after reset but Release do not. Sometimes even Debug binaries do not work after reset.
CircuitPython works fine because it uses PICO_FLASH_SPI_CLKDIV of 4 (confirmed by disassembling the binary).
I think PICO_FLASH_SPI_CLKDIV should be set to 4 in SDK for this board.

[AlynxZhou]

I meet this problem too with 2 of 3 RP2040-Zeros I bought, a interesting thing is I found setting PICO_XOSC_STARTUP_DELAY_MULTIPLIER to 64 also fixes this bug, I think it makes program waits longer before startup.

But your words are more reasonable, and I tried your suggestion, it does fix the bug, so I submit a PR according to this issue. Thanks!

[lurch]

In case it's useful, see the related discussion in #401

[AlynxZhou]

In case it's useful, see the related discussion in #401

Thanks!

My test shows that only change PICO_FLASH_SPI_CLKDIV to 4 is enough, there is no need to change both PICO_FLASH_SPI_CLKDIV and PICO_XOSC_STARTUP_DELAY_MULTIPLIER.

[kilograham]

merged into develop