Hey Folks
I'm building a data aquisition unit consiting of three Arduino Mega2560. Currently I'm just trying to make two of them talk to each other with SPI.
The slave is going to read out the sensors and then send the data to the Master. Since i can only send 1 byte at a time, I take the sensor value apart, send two bytes and put it back together on the Master side.
I read up on SPI and as far as I understand the SPI.transfer sends and receives simultaneously. Therefore I let the Master initiate a transfer with a specific command byte which calls a function on the slave. The function sends back the data to the Master.
Here is the code so far:
Master
#include "SPI.h"
#define SS_PIN 53
SPISettings mySettings;
void useClockSpeed(unsigned long clock) {
mySettings = SPISettings(clock, MSBFIRST, SPI_MODE3);
}
int getSensor(void) {
SPI.beginTransaction(mySettings);
digitalWrite(SS_PIN, LOW);
// Send start Byte
SPI.transfer('s');
delay(100);
// Get first Byte
byte first = SPI.transfer('1');
delay(100);
// Debugging
Serial.println("First Byte: ");
Serial.println(first);
// Get second Byte
byte second = SPI.transfer(2);
delay(100);
// Debugging
Serial.println("Second Byte: ");
Serial.println(second);
// End transmission
digitalWrite(SS_PIN, HIGH);
SPI.endTransaction();
// Calculating actual sensor data
Serial.println("Actual Data:");
int value = (first << 8) | second;
// Debugging
Serial.println(value);
return value;
}
void setup() {
pinMode(SS_PIN , OUTPUT);
digitalWrite(SS_PIN, HIGH);
Serial.begin(57600);
SPI.begin(); // wake up the SPI bus.
SPI.setBitOrder(MSBFIRST);
}
void loop() {
// Will later be a for loop to get senor data of several sensors
getSensor();
}
Slave
#include "SPI.h"
#define SCK_PIN 52
#define MISO_PIN 50
#define MOSI_PIN 51
#define SS_PIN 53
#define Sensor1 A0
void SlaveInit(void) {
// Set MISO output, all others input
pinMode(SCK_PIN, INPUT);
pinMode(MOSI_PIN, INPUT);
pinMode(MISO_PIN, OUTPUT);
pinMode(SS_PIN, INPUT);
// Enable SPI
SPCR = B00000000;
SPCR = (1 << SPE);
}
void SensorTransfer(int Pin) {
//int Sensor = analogRead(Pin);
//Dummy value until transfer works and actual sensors are hooked up
int Sensor = Pin;
// Shifts 8 positions to right. Here the number 2 results
byte one = Sensor >> 8;
// transmit first byte
byte rx1 = SPI.transfer(one);
// Debugging
Serial.print("First receive: ");
Serial.println(rx1);
// Getting second part of the number. Here it is 108
byte two = Sensor & 0xFF;
// transmit second byte
byte rx2 = SPI.transfer(two);
// Debugging
Serial.print("Seconde receive: ");
Serial.println(rx2);
}
void setup() {
Serial.begin(57600);
SPI.begin();
SlaveInit();
}
void loop() {
SlaveInit();
// Wake up slave and execute command
byte select = SPI.transfer(1);
// Debugging
Serial.print("Got Select byte: ");
Serial.println(select);
// If Master asks for Sensor Transfer - do so
if(select == 's'){
SensorTransfer(620);
}
However I have some timing problems with the whole SPI.transfer business.
With this code i get this on the serial monitor most of the time
First Byte:
2
Second Byte:
108
Actual Data:
620
However i seem only to get this correct readout when the delays in the masters getSensor function are all set to 100. With other values i either get the bytes swaped resulting in the wrong actual data number, the numbers sent get echoed back or sometimes even utter nonsense.
How do I tackle the problem that I always get the right 'pairs' of SPI.transfer to talk to each other? And also how can it be done faster. Since I'm aiming on getting 200Hz readouts with 8 sensors hooked up on each Arduino, a delay of 100ms like in the code wouldn't work at all.
On this note just one more question: is it even possible to analogRead 8 sensors and send the 16bytes back to the Master in less than 5ms (200Hz)?
Best Regards,
Morux