pic Interrupt on change doesn't trigger first time - interrupt

Question

I'm trying to comunicate a pic18f24k50 with an arduino 101. I'm using two lines to establish a synchronized comunication. On every change from low to high of the first line, I read the value from the second line.
I have no problems with the arduino code, my problem is that in the pic, the Interrupt on change triggers on the second change from low to high instead of triggering on the first change. This only happens the first time I send data, after that, it works perfectly (it triggers on the first change and I receive the byte properly). Sorry for my english, I'll try to explain myself better with this image:
Channel 1 is the clock signal, channel2 is the data (Im sending one byte for the moment, with bit values 10101010). Channel 4 is an output I'm changing every time I process a bit. (as you can see, it begins on the second rise of the clock signal instead of the first one). This is captured on the first sent byte, the next ones works ok.
I post the relevant code in the pic here:
This is where I initialize things:
TRISCbits.TRISC6 = 0;
TRISCbits.TRISC1 = 1;
TRISCbits.TRISC2 = 1;
IOCC1 = 1;
ANSELCbits.ANSC2=0;
IOCC2 = 0;
INTCONbits.IOCIE = 1;
INTCONbits.IOCIF = 0;
And this is on the interrupt code:
void interrupt SYS_InterruptHigh(void)
{
if (INTCONbits.IOCIE==1 && INTCONbits.IOCIF==1)
{
readByte();
}
}
void readByte(void)
{
while(contaBits<8)
{
INTCONbits.IOCIE = 0;
INTCONbits.IOCIF = 0;
while (PORTCbits.RC1 != HIGH)
{
}
if (PORTCbits.RC1 == HIGH)
{
LATCbits.LATC6 = !LATCbits.LATC6;
//LATCbits.LATC6 = ~LATCbits.LATC6;
switch (contaBits)
{
case 0:
if (PORTCbits.RC2 == HIGH)
varByte.b0 = 1;
else
varByte.b0 = 0;
break;
case 1:
if (PORTCbits.RC2 == HIGH)
varByte.b1 = 1;
else
varByte.b1 = 0;
break;
case 2:
if (PORTCbits.RC2 == HIGH)
varByte.b2 = 1;
else
varByte.b2 = 0;
break;
case 3:
if (PORTCbits.RC2 == HIGH)
varByte.b3 = 1;
else
varByte.b3 = 0;
break;
case 4:
if (PORTCbits.RC2 == HIGH)
varByte.b4 = 1;
else
varByte.b4 = 0;
break;
case 5:
if (PORTCbits.RC2 == HIGH)
varByte.b5 = 1;
else
varByte.b5 = 0;
break;
case 6:
if (PORTCbits.RC2 == HIGH)
varByte.b6 = 1;
else
varByte.b6 = 0;
break;
case 7:
if (PORTCbits.RC2 == HIGH)
varByte.b7 = 1;
else
varByte.b7 = 0;
break;
}
contaBits++;
}
}//while(contaBits<8)
INTCONbits.IOCIE = 1;
contaBits=0;
}
LATCbits.LATC6 = !LATCbits.LATC6; <-- this is the line corresponding to channel 4.
RC1 is channel 1
and RC2 is channel 2
My question is what am I doing wrong, why on the first sent of bytes the interrupt doesn't triggers on the first change of the line 1?
Thank you.

Answer 1

What are you trying to achieve?
The communication protocol as you're describing is commonly refered to as SPI (Serial Peripheral Interface). You should use the hardware implementation available by PIC/Microchip, if possible, for best performance.
Keep your code documented/formatted/logic
I noticed your code being a little, weird.
Weird code gives weird errors.
First:
You're using interrupts, you have this blocking code in your interrupt. Which isn't nice at all.
Second:
Your "contabits" and "varByte" variable come out of nowhere, probably they're global. Which might not be the best practice.
Though, if you're using interrupts, and when it might make sense to transfer the variables to your main program by using a global. The global should be volatile.
Third:
That case switch is just 8x the same code, but a little different.
Also, add some comments to your code.
To be honest
I didn't spot your actual error, has been a long time since I worked with PIC.
But, you should try hardware SPI.
Below is my attempt at software SPI as you described, it might be a little more logical, and takes out any annoyance of the interrupts.
I would recommend replacing the while(CLOCK_PIN_MACRO != 1){}; with for(unsigned int timeout = 64000; timeout > 0; timeout--){};.
So that you can be sure your program won't hang while waiting for an SPI signal that won't come. (Make the timeout something what fits your needs)
#define CLOCK_PIN_MACRO PORTCbits.RC1
#define DATA_PIN_MACRO PORTCbits.RC2
/*
Test, without interrupts.
I would strongly advise to use Hardware SPI or non-blocking code.
*/
unsigned char readByte(void){
unsigned char returnByte = 0;
for(int i = 0; i < 8; i++){ //Do this for bit 0 to 7, to get a complete byte.
while(CLOCK_PIN_MACRO != 1){}; //Wait until the Clock pin goes high.
if(DATA_PIN_MACRO == 1) //Check the data pin.
returnByte = returnByte & (1 << i); //Shift the bit in our byte. (https://www.cs.umd.edu/class/sum2003/cmsc311/Notes/BitOp/bitshift.html)
while(CLOCK_PIN_MACRO == 1){}; //Wait untill the Clock pin goes low again.
}
return returnByte;
}
void setup(){
TRISCbits.TRISC6 = 0;
TRISCbits.TRISC1 = 1;
TRISCbits.TRISC2 = 1;
//IOCC1 = 1;
ANSELCbits.ANSC2=0;
//IOCC2 = 0;
INTCONbits.IOCIE = 1;
INTCONbits.IOCIF = 0;
}
void run(){
unsigned char theByte = readByte();
}
main(){
setup();
while(1){
run();
}
}

Answer 2

You can find the following from the datasheet.
The pins are compared with the old value latched on the last read of
PORTB. The "mismatch" outputs of RB7:RB4 are ORed together to generate
the RB Port Change Interrupt with Flag bit,RBIF (INTCON<0>).
This means that when you read PORTB RB7:RB4 pins, the value of each pin will be stored in an internal latch. Interrupt will be generated only when there is any change in the pin input from the previously latched value.
Then, what will be the default initial value in the latch? i.e. Before we read anything from PORTB.
What ever it is. in your case the default latch value is different from the logic state of your input signal. During the next transition of the signal the latch value and signal level becomes same. So no intterrupt is generated first time.
What you can do is setting the latch value to the initial state of your signal. This can be done by reading PORTB (should be done before enabling the intterrupt).
The below code is enough to simply read the register.
unsigned char ch;
ch = PORTB;
Hope this will help.