Solved: Programs with SPI connection to RPi won't start

Questions » Solved: Programs with SPI connection to RPi won't start

Jeroen Voogd

10 years ago.

Hi, relative MCU-newbie here ...

I have a Raspberry Pi connected to a NXP LPC812-max via SPI. I got it working ... with some slight of hand.

Set-up: the 4 SPI + GND connections between the RPi and the LPC812 are in place (directly connected). The RPi runs the following SPI-master code:

Code for RPi SPI master

import spidev
import time

spi = spidev.SpiDev()
spi.open(0,0)

try:
    while True:
        for i in range(256):
            send = [i]
            print "RPi SPI sending:", send
            resp = spi.xfer2(send)
            print "RPi received:", resp[0]
            time.sleep(1)
except KeyboardInterrupt:
    spi.close()

The LPC812 runs the following SPI-slave code:

Code for LPC812-max SPI slave

#include "mbed.h"

int main()
{  Serial serial(USBTX, USBRX);    // open serial communication
    serial.printf("Mbed getting ready");

    SPISlave device(P0_14, P0_15, P0_12, P0_13); // mosi, miso, sclk, css
    device.format(8,0);             // bits=8, mode=0
    device.reply(0x21);             // Prime SPI with first reply
    while(1) {
        if(device.receive()) {
            int v = device.read();  // Read byte from master
            v = v & 0xFF;           // Only take the lower 8 bits
            serial.printf("Mbed has received: %d\r\n",v);
            v = (v + 1) % 0x100;    // Add one to it, modulo 256
            serial.printf("Mbed is going to send: %d\r\n",v);
            device.reply(v);        // Make this the next reply
        }
    }
}

If I start the RPi code and do a reset on the LPC812 nothing happens. I have to disconnect the SCK, do a reset and re-insert the SCK before everything runs as it should. Also if I load another program such as a blinking led, it won't start until I disconnect, reset and re-connect the SCK.

What am I doing wrong?

Wim Huiskamp provided the answer: the LPC812 version C4 (which is indeed on the LPC812-max board I use) has the ISP entry pin on PI00_12 (P0_12), which is also used for the SPI SCK. The SPI SCK in mode 0 will (almost always) cause the board to expect a program to be flashed after a reset. So, I changed the SPI mode to 2 (inverted SCK) and now my program starts (almost always).

posted by Jeroen Voogd 09 Dec 2014

Note that this problem will bite you whenever you use P0_12 as DigitalIn (or even DigitalOut) pin that could be at a Low level during a hard reset or power-on reset! The LPC812 will then enter the ISP bootmode and expects to be flashed using the serial port rather than start normally.

posted by Wim Huiskamp 14 Dec 2014

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Jeroen Voogd

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