Eye SPI Arduino…

I’ve been toying with how to handle multiple peripherals that require Serial. Up until now I’ve used the Software Serial library on Arduino, and have resorted for my SD card logger to use the hardware serial.

But is there a better way…

Maybe…

Lets have a look at the comms options:-

Part	Hard Serial	Soft Serial	SPI	I2C
ArduLogger V3	Y	NO	YES [0]	YES [0]
GPS – GP635T	Y	Y	NO	NO
GPS – UBlox 6M module	Y	Y	NO	NO [1]
3DR Radio	Y	Y	NO	NO
LCD – Hitachi 44780	Y	Y	NO [2]	NO

[0] ArduLog software only, not the SparkFun OpenLog software. Oh and you need to modify the code yourself to add support!
[1] The compass on the UBlox does have I2C SCL and SDA lines, but not the GPS
[2] It is possible to use a register to drive the LCD. May be possible to cleverly link this to an SPI interface

Hmmm… So maybe not then… Although of course nothing stopping me using interface circuitry to make all the above work, but it would probably add to the number of IO lines used, not reduce them!

Speaking of IO lines used, I’m currently using these:-

• ArduLogger – 2 (hardware serial)
• GP635T – 3 (2x soft serial, 1x power mosfet on/off)
• 3DR Radio – 3 (2x soft serial, 1x power mosfet on/off)
• LCD – 6
• TOTAL: 14 (Arduino Pro Mini V3 I use has 22 pins that can be used for digital IO – 4 of these are optionally SPI, and another two optionally I2C)

I’ll keep a watching brief on using SPI though – could potentially be useful, and allow me to ‘off board’ a lot of serial comms in future.

[UPDATE 26 MAY 2015]

You can buy ICs that act as an SPI to multiple UART convertors each with a 64 byte FIFO queue. The chip model for two output UARTs (up to 4MBit/s!) is

SC16IS752IBS and is documented at http://www.nxp.com/documents/leaflet/75015676.pdf. This product also has some GPIO ports, so could theoretically externalise all my IO (including LCD) if I really needed to.

Hobbytronics ArduLogger V3 and Arduino pro mini circuit…

I’ve got a nice simple working circuit for my ArduLogger V3 board. Re-jigged it just now to minimise the number of wires. Image below for your edification:-

Ardu Logger V3 and Arduino Pro Mini

Remember too, when using a solderable breadboard (from SparkFun) of the same size, the logging sd card board will rotate 180 degrees, and sit below the breadboard rather than sticking out. A nice compact logging circuit.

I’ve got some test software that simulates logging GPX (XML) format GPS based track information to the SD card every couple of seconds. Sample SD card append code is available on my GitHub page.

Connect the FTDI to the pins (on the right of the breadboard, above) to your computer, and you’re away!

BE AWARE: When programming the board, remove the ArduLogger breakout from the circuit, else it interferes with serial communication to the Arduino pro mini. Once reprogrammed, disconnect the FTDI cable, add the ArduLogger back in, then reconnect the FTDI (to power the circuit). Alternatively power the breadboard directly.

Note on wires above:-

• Blue wire links ground on the logger to the Arduino
• yellow, orange, and grey wires take a circuitous route to link 5V with VCC on the Arduino
• The position of the logger board next to the Arduino automatically lines up and connects TX and RX pins (and RST)

The sharp eyed among you may have noticed the Arduini is a 3.3V 8MHz variant, whereas the power pin on the logger says 5V. You can run 3.3V through here quite happily.

Note that as previously mentioned, the ArduLogger board and the open logger software only works with Hardware Serial – software serial WILL NOT work. You have been warned.