Background: micro-gui test scripts displayed nothing when located in external flash. The code was evidently running, but the screen was blank. The scripts displayed when the display driver was located in the filesystem with all the rest of the code in spi flash. With all code except the driver in the filesystem, the script worked correctly if the driver was frozen normally, but failed to display if the driver was located in external flash.

The workround is simply to put the (small) driver in the filesystem but the fault appears to be symptomatic of a firmware issue.

When failing, the SPI transfer which occurs on initialisation was garbled, occurring as short incorrect bursts in place of one long sequence. The following test script demos the problem. Note that occasionally it runs correctly, but it usually fails.

import framebuf
import time
import gc
from machine import Pin, SPI

pp = Pin('EN_3V3')
pp(1)
time.sleep(1)

pdc = Pin('X1', Pin.OUT_PP, value=0)
pcs = Pin('X2', Pin.OUT_PP, value=1)
prst = Pin('X6', Pin.OUT_PP, value=1)
spi = SPI(2, baudrate=2_000_000)
gc.collect()  # Precaution before instantiating framebuf

class SSD1351(framebuf.FrameBuffer):

    def __init__(self, spi, pincs, pindc, pinrs, height=128, width=128):
        self.spi = spi
        self.pincs = pincs
        self.pindc = pindc  # 1 = data 0 = cmd
        self.height = height  # Required by Writer class
        self.width = width
        mode = framebuf.GS8  # Use 8bit greyscale for 8 bit color.
        gc.collect()
        self.buffer = bytearray(self.height * self.width)
        super().__init__(self.buffer, self.width, self.height, mode)
        self.linebuf = bytearray(self.width * 2)
        pinrs(0)  # Pulse the reset line
        time.sleep_ms(1)
        pinrs(1)
        time.sleep_ms(1)
        self._write(b'\xfd\x12\xfd\xb1\xae\xb3\xf1\xca\x7f\xa0\x74'\
        b'\x15\x00\x7f\x75\x00\x7f\xa1\x00\xa2\x00\xb5\x00\xab\x01'\
        b'\xb1\x32\xbe\x05\xa6\xc1\xc8\x80\xc8\xc7\x0f'\
        b'\xb4\xa0\xb5\x55\xb6\x01\xaf', 0)
        pinrs(0)  # For stand-alone test: pins are unconnected.
        pinrs(1)
        gc.collect()
        time.sleep(1)

    def _write(self, buf, dc):
        self.pincs(1)
        self.pindc(dc)
        self.pincs(0)
        self.spi.write(buf)
        self.pincs(1)

ssd = SSD1351(spi, pcs, pdc, prst)

I am writing to external flash with the following change to f722_qspi.ld and doing a clean build. The Pyboard is attached to no external hardware.

/* Define output sections */
SECTIONS
{
    .text_ext :
    {
        . = ALIGN(4);
        *frozen_content.o(.text* .rodata*)
        *lib/btstack/*(.text* .rodata*)

The following images show the behaviour where the transfer starts then prematurely terminates. No further data is transferred. Behaviour in the actual application is more complex, with further brief transfers with unexpected data occurring later.

For comparison this is the start of a successful run:

Zooming out of the failing run:

Firmware:

MicroPython v1.18-355-g9ab66b50c-dirty on 2022-04-29; PYBD-SF2W with STM32F722IEK