#216 OSHChip/LEDx16Module

Driving an SPI LED module with the OSHChip.

LEDx16Module_build

Notes

How easy is SPI with the OSHChip? I thought I’d find out by first controlling a module that has a very basic SPI-ish slave interface.

I’m using the LEDx16Module that I designed in the KiCad like a Pro course from Tech Explorations. It has dual 74HC595 shift registers that can be driven with SPI to control 16 onboard LEDs.

SPI Driver Options

It seems there’s at least three options:

use the SPI class that is part of the mbed standard library
go direct to registers for SPI control
pick one of the other SPI libraries out there, for example SPI_Demo_Nano that implements an SPI class very similar to the Arduino SPI library.

I’m going to code this one with the mbed online compiler, so I’ll start with the first option.

Configuring and Using SPI

I’ll probably publish my project on mbed, but for now the source is all here. The demo code is largely copied from the LEDx16Module demo.

It sets SPI for an 8 bit data frame, spi mode 0, 1MHz clock rate:

// Setup spi: 8 bit data frame, spi mode 0, 1MHz clock rate
spi.format(8, 0);
spi.frequency(1000000);

And 16-bits worth of data (for the two shift registers) is sent in two write operations:

led_module_cs = 0;
spi.write(data);
spi.write(data >> 8);
led_module_cs = 1;

Two quirks that I still don’t fully understand:

I thought IO should be able to set a 16-bit frame with format(16, 0) but I couldn’t get it to work.
even though SPI.write takes an “int”, it still only sends the 8 lower bits, hence the transfer in two calls

Also note I haven’t resorted to bulk transfers using the transfer method in this case, since I’m not sending large quantities of data, and not expecting any reply.

Construction

There’s not much to the build. Basically hook together three modules:

the OSHChip
the LEDx16Module
3.3V power supply - I’m using a Buck Converter Module

LEDx16Module_bb

LEDx16Module_schematic