Fast, small, and simple Arduino SPI RAM chip routines.

The 8-pin chip, 23LC1024, is a good candidate for adding extra RAM to simple Arduinos such as the Uno, Nano, Mini.  It provides 128K of RAM (the ATmega328 chip that these Arduinos use only has 2K of internal RAM).

The chip works at 5V (anything from 2.5V to 5.5V) and uses SPI so it only uses 4 pins of the Arduino to talk to it.  You can buy the chips on eBay for less than £5 each.

I wanted some fast simple routines to copy any arbitrary number of bytes between ‘normal’ RAM and the SPI RAM.  I didn’t want to link in any huge libraries so I wrote these small routines that hit the 328 chip’s SPI registers direct.

There are only three functions:

spiRam::start(); // sets up the SPI interface – call once from your setup() routine

spiRam::writeSpi(*ptr, address, length); // copies to SPI RAM

spiRam::readSpi(*ptr, address, length); // copies from SPI RAM

*ptr is an unsigned byte (uint8_t) pointer – but you can cast any other data type address to that easily – see example.

address is an unsigned long (uint32_t) as addresses in the SPI RAM range from 0 to 131071 (2^17 – 1).

length is an unsigned int (uint16_t) – but will never be bigger than 2048 as the 328 chip only has 2K of RAM.

Standard connections (you can use a different pin than D10 for the chip select – but there’s not much point unless you want to use more than one RAM chip to give more than 128K RAM – see notes in the .h file).

Here’s the sketch: SpiRam23LC1024.zip Unzip it inside your Arduino folder. You want to end up with the SpiRam23LC1024 folder in your Arduino folder with the SpiRam23LC1024.ino and the two other files inside.  Then it should compile and upload okay.  To use it with your own code just move the .h and .cpp files inside your folder where your .ino file is and add the line

#include “spiRam.h”

up at the top of your sketch.

Because the routines are interrupt driven your code could be doing something else while the transfers are taking place – but be careful if you’re not sure – it’s easy to start destroying or changing contents before they’ve finished being transferred if you’re careless. The routines are very fast anyhow (9 milliseconds to transfer 1024 bytes in either direction), so as standard there is a ‘busy wait loop’ until the transfers are complete so you don’t need to worry about checking from inside your code.