Arduino SPI hangs during transfers - c++

Question

I'm in the middle of a project, and at a loss. I managed to get 2 Arduinos to talk to each other over SPI, but the slave stops during a series of transfers, seemingly without reason.
And what's really disturbing is that if I send more transfers from the master it continues the series, as if it was waiting to continue.
I'm thinking there's some acknowledgement or flag that stops code execution, but I have no idea.
#include <SPI.h>
boolean ack = 0;
#define ACK 2
byte buffer = 0;
byte rx = 0;
bool SSlast = HIGH;
byte clr = 0;
void stat_upd(byte dat, byte ric) {
Serial.println("---------------------------------------------");
Serial.println("Sent:");
Serial.println(dat, HEX);
Serial.println("Received:"); // 0x81 in teoria
Serial.println(ric, HEX);
return;
}
// Initialize SPI slave.
void SlaveInit(void) {
// Initialize SPI pins.
pinMode(SCK, INPUT);
pinMode(MOSI, INPUT);
pinMode(MISO, INPUT);
pinMode(SS, INPUT);
pinMode(ACK, OUTPUT);
// Enable SPI as slave.
SPCR = 0x6F;
clr = SPSR;
clr = SPDR;
SPI.begin();
}
// SPI Transfer.
byte SPItransfer(byte value) {
byte temp = 0;
SPDR = value;
// temp =SPI.transfer(value);
while (!(SPSR & (1 << SPIF)));
digitalWrite(ACK, LOW);
delay(1);
digitalWrite(ACK, HIGH);
delay(10);
return SPDR;
}
// The setup() function runs after reset.
void setup() { ///////////////// setup
Serial.begin(9600);
SlaveInit();
Serial.println("MC Initialized");
}
void loop() { ////////////// loop
// Slave Enabled?
if (!digitalRead(SS)) {
rx = SPItransfer(0x00);
stat_upd(0x00, rx);
rx = SPItransfer(0x08);
stat_upd(0x08, rx);
rx = SPItransfer(0x5a);
stat_upd(0x5a, rx);
rx = SPItransfer(0x5d);
stat_upd(0x5d, rx);
rx = SPItransfer(0x5c);
stat_upd(0x5c, rx);
rx = SPItransfer(0x5d);
stat_upd(0x5d, rx);
rx = SPItransfer(0x04);
stat_upd(0x04, rx);
rx = SPItransfer(0x00);
stat_upd(0x00, rx);
rx = SPItransfer(0x00);
stat_upd(0x00, rx);
rx = SPItransfer(0x80);
stat_upd(0x80, rx);
}
}

Answer 1

This is a bit of a guess, but my guesses wrt Arduino code are correct more often then not.
Check how you are sending the data via the Master.
As per this code, the slave waits for the SS pin to go low, then talks to the SPI and then clears the ack pin for 1 millisecond and then sends the communicaiton data via the serial port to the PC.
The problem I see here is that 1 millisecond is too small of a time frame for the master to detect it correctly.
So my guess is that, you have written the master code in such a manner, that it does not look at the ACK pin. Which means that the master will clear the SS pin, do the communication and then clear the SS pin immediately.
This is an issue because the slave talks to the serial port once this communication is done. Which means that when the master sets the SS pin low again, the slave might be sending data to the serial monitor, and may miss out on the communication altogether.
To fix this you need to change the slave code to continuously talk to the master while the SS pin is low, and only send the data back to the serial monitor when the SS pin becomes high.