How to read Arinc message converted to I2C (?) on an Arduino

[ajakobsson]

Hi

Can anyone guide me in the right direction how to read a "non standard" I2C word?

I've got a Holt QSG-8476 Arinc to paralell and SPI converter evaluation board, the datasheet claims it's SPI, but as far as I can see it's more I2C than SPI.

Link to the "evaluation" board: Download document 'QSG-8476' - Holt Integrated Circuits, Inc.
Link to the chip: Download document 'HI-8475' - Holt Integrated Circuits, Inc.

I've tried several solutions both with I2C and SPI, but only the I2C has gotten me anywhere.
Problem is, I'm only getting either 32 bits containing all 1s or a single 0

Attached picture shows a scope image of whats being sent to Arduino.

My code (just to see if there's anything usable coming in):

#include <Wire.h>

int32_t receivedWord; //32 bit "word" sent from Arinc converter


void setup() {

  Serial.begin(115200);
  Wire.begin();


}

void loop() {
  
int32_t recievedWord = Wire.read(); //Read the Arinc word
  
  Serial.println(receivedWord, BIN); 
    
    delay(500);
  
}

I've also tried setting the Arduino up as a slave by stating the first 7 bits of the Arinc word as its address (0x68) (Arinc transmits "Label number" first, stated in octal, so trying to read ex. label 320 "Magnetic heading" the first bits from Arinc would be 11010000.)

[pylon]

I've tried several solutions both with I2C and SPI, but only the I2C has gotten me anywhere.

That interface is neither I2C nor SPI. You must not connect those signal to your Arduino's SPI or I2C pins while using those libraries or you might damage your Arduino and/or the Holtic board.

The best match is SPI slave mode using the UPDATE pin to drive SS, but that's only the first impression while reading the datasheet. That way you don't have a short-circuit at least.

[blh64]

SCLK and SDAT is just the clock/data passing through the board. Look at figure 2 of the chip document. Also look at Figure 6 to see the timing. On each rising edge of SCLK, SDAT is valid.

If this is what you want to capture, you will have to watch for that and sample SDAT and build up your 32 bit word.