# SPI write crashes the microbit. I'm baffled

**URL:** <https://forum.makecode.com/t/spi-write-crashes-the-microbit-im-baffled/2091>\
**Category:** micro:bit\
**Created:** [May 14, 2020, 7:27pm UTC](https://forum.makecode.com/t/spi-write-crashes-the-microbit-im-baffled/2091 "2020-05-14T19:27:56Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![mkilpatr](https://avatars.discourse-cdn.com/v4/letter/m/ccd318/32.png) [@mkilpatr](https://forum.makecode.com/u/mkilpatr)\
**Post date:** [May 14, 2020, 7:27pm UTC](https://forum.makecode.com/t/spi-write-crashes-the-microbit-im-baffled/2091/1 "2020-05-14T19:27:56Z")

</div>

I have a situation in which a write to SPI seems to crash the Microbit. I’ve been looking at this for an hour or two now. I really cannot see what is wrong.

My Makecode extension to drive a Nokia 3310 LCD screen (84 x 48 pixels) is in this repository  
[https://github.com/MTKilpatrick/pxt-nokialcd](https://github.com/MTKilpatrick/pxt-nokialcd)

The TS file has shims for lots of functions that I have transferred to the .cpp file to speed up all the graphics. I’m transferring all the SPI routines to C as well.

The extension beings by calling `init()` in JS which in turn calls` SPIinit()` (in the CPP) to initialise the SPI port. `init()` then sets up the LCD display with several commands - these are determine by setting Digital Pin16 to low (LCD\_CMD). Data writes to the screen are performed when P16 is high (LCD\_DAT). The chip select is performed by P12 and reset by P8.

The first set-up call is `writeFunctionSet()` then ` lcdExtenedFunctions()` which sets the display into a command mode in order to set up a few parameters. `writeFunctionSet()` exists in both the JS and C files.

This extension makes the Microbit hang up if I use the `shim=nokialcd::writeFunctionSet` to make it use the C version of writeFunctionSet(). If I use the JS version of `writeFunctionSet()` by removing the shim, the whole thing works.

If you look at the code in both the TS and CPP files, `writeFunction()` is identical. It merely sets P12 and P16 to the appropriate states, calls `spi.write()` with a value and then reverts P12 and P16.

Please note that spi.write() is also used in the function `writeSPIBuf()` in the CPP file - this transfers the 504-byte buffer to the screen. This function works and I have tested it.

What is going on here? If I comment out the `spi.write(0x20 | (v << 1) | (h & 1));` line in writeFunctionSet, the init() routine completes. If I leave the line it, the Microbit hangs up at that point.

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<div class="post-metadata">

**Author:** ![mkilpatr](https://avatars.discourse-cdn.com/v4/letter/m/ccd318/32.png) [@mkilpatr](https://forum.makecode.com/u/mkilpatr)\
**Post date:** [May 14, 2020, 7:50pm UTC](https://forum.makecode.com/t/spi-write-crashes-the-microbit-im-baffled/2091/2 "2020-05-14T19:50:25Z")

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Update: The `spi.write()` function in C only works if, beforehand, I have done at least one call to `pins.spiWrite()` in the JS code.

I tested this by removing all of the set-up calls in` init()` after initialising the SPI port and then making a call to `writeSPIBuf()` (in C) to push the 504-byte buffer to the screen. This operation did not complete. It only completed after I uncommented a minimum of one JS function call that writes a byte to the SPI.

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<div class="post-metadata">

**Author:** ![mkilpatr](https://avatars.discourse-cdn.com/v4/letter/m/ccd318/32.png) [@mkilpatr](https://forum.makecode.com/u/mkilpatr)\
**Post date:** [May 14, 2020, 11:13pm UTC](https://forum.makecode.com/t/spi-write-crashes-the-microbit-im-baffled/2091/3 "2020-05-14T23:13:18Z")

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OK, so this Nokia LCD is supposed to use spi.Format(8,0) not (8,3). But it works on either presumably because of the timing on MOSI. Anyway…

When I successfully no longer have SPI write operations in the Javascript having moved them all to C, I find that the thing crashes at the` init()` stage unless I put _either_ or _both_ a `spiFormat()` or `spiFrequency()` calls in the `init()` function in Javascript:

```
export function init(): void {
    pins.spiFormat(8, 0)
    pins.spiFrequency(1000000)
    SPIinit()
}

```

Even though in the CPP file I have this:

```
namespace nokialcd {
   SPI spi(mbit_p15, mbit_p14, mbit_p13);
    DigitalOut LCD_CE(mbit_p12);
    DigitalOut LCD_RST(mbit_p8);
    DigitalOut LCD_DC(mbit_p16);
    static Buffer bytearray = NULL;
    static bool state = true;
    static int lcdDE = 0;
   //%
       void SPIinit() {
        LCD_CE = 1;
        lcdDE = 0;
        LCD_RST = 0;
        spi.format(8,0);
        spi.frequency(1000000);
....and so on....

```

Which clearly inititalises the SPI device. Why does it need to be _either_ formatted _or_ frequency-set in the JS too? That makes no sense to me.