Hello, I'm trying to get 2 arduinos to talk to each other over SPI. Now it works just fine, as long as SCK is at 1MHz or lower (DIV16, DIV32, DIV64 or DIV128).
My program is simple, the master sends a string of numbers (0-255) and the slave prints these numbers out in hex, checking them to ensure all bytes have been recieved. If there is a mismatch, it stops.
At 1MHz, it will happily run for hours without a mismatch, but anything higher causes a mismatch within less than a second.
Has anyone got any advice on properly terminating the SCK line? currently I just have jumper wires (very short, not more than 6cm) between the two arduinos.
Surely impedance matching isn't necessary for such short distance/low freq?
Hello, I'm trying to get 2 arduinos to talk to each other over SPI. Now it works just fine, as long as SCK is at 1MHz or lower (DIV16, DIV32, DIV64 or DIV128).
My program is simple, the master sends a string of numbers (0-255) and the slave prints these numbers out in hex, checking them to ensure all bytes have been recieved. If there is a mismatch, it stops.
At 1MHz, it will happily run for hours without a mismatch, but anything higher causes a mismatch within less than a second.
Has anyone got any advice on properly terminating the SCK line? currently I just have jumper wires (very short, not more than 6cm) between the two arduinos.
Surely impedance matching isn't necessary for such short distance/low freq?
Hello, I'm trying to get 2 arduinos to talk to each other over SPI. Now it works just fine, as long as SCK is at 1MHz or lower (DIV16, DIV32, DIV64 or DIV128).
My program is simple, the master sends a string of numbers (0-255) and the slave prints these numbers out in hex, checking them to ensure all bytes have been recieved. If there is a mismatch, it stops.
At 1MHz, it will happily run for hours without a mismatch, but anything higher causes a mismatch within less than a second.
Has anyone got any advice on properly terminating the SCK line? currently I just have jumper wires (very short, not more than 6cm) between the two arduinos.
Surely impedance matching isn't necessary for such short distance/low freq?
yup it does, and although the SPI interface uses this same clock signal, you can set a divider to scale it down. works fine with a divider of 16, meaning the freq is 1MHz.
But I start to lose bytes at frequencies above this. Has anyone successfully received SPI data on an arduino at > 1MHz?
eriknyquist:
Surely impedance matching isn't necessary for such short distance/low freq?
Nope.
I suspect something like an interrupt coming along and using up more time than it takes two bytes to arrive over SPI, losing one of them.
Try doing "cli()". This will disable all interrupts. millis(), delay(), etc. will stop working but at least you'll know if that's the problem so you know where to start looking.
How are you printing the numbers out? SPI waits for no man, if you a dicking around with serial printing and converting binary to ASCII HEX you may well lose data.
How are you storing the numbers?
Are you using interrupts for the SPI?
I think you should post both halves of the code, there's little point commenting without all the facts.
Surely impedance matching isn't necessary for such short distance/low freq?
Sorry for cross-posting guys. Didn't realise it was a no no- won't happen again!
Heres my master and slave code anyway.
[code]
MASTER:
#include <SPI.h>
void setup (void)
{
unsigned int i;
digitalWrite(SS, HIGH); // ensure SS stays high for now
// Put SCK, MOSI, SS pins into output mode
// also put SCK, MOSI into LOW state, and SS into HIGH state.
// Then put SPI hardware into Master mode and turn SPI on
SPI.begin ();
Serial.begin(9600);
// Slow down the master a bit
SPI.setClockDivider(SPI_CLOCK_DIV8);
// enable Slave Select
} // end of setup
void loop (void)
{
unsigned int i;
digitalWrite(SS, LOW); // SS is pin 10
// send test string
//for (const char * p = "Hello, world!\n" ; c = *p; p++)
// SPI.transfer (c);
for(i=0;i<250;i++){
SPI.transfer (i);
}
// SPI.transfer (c);
// disable Slave Select
digitalWrite(SS, HIGH);
delay (2000); // 1 seconds delay
byte a = SPDR;
Serial.println(a);
} // end of loop
SLAVE:
#include <SPI.h>
unsigned char buf [300];
volatile byte pos;
volatile boolean process_it;
char p;
void setup (void)
{
Serial.begin (9600); // debugging
// have to send on master in, *slave out*
pinMode(MISO, OUTPUT);
// turn on SPI in slave mode
SPCR |= _BV(SPE);
// get ready for an interrupt
pos = 0; // buffer empty
process_it = false;
// Slow down the master a bit
SPI.setClockDivider(SPI_CLOCK_DIV8);
// now turn on interrupts
SPI.attachInterrupt();
//SPI.setDataMode()
} // end of setup
// SPI interrupt routine
ISR (SPI_STC_vect)
{
byte c = SPDR; // grab byte from SPI Data Register
// add to buffer if room
if (pos < sizeof buf)
{
buf [pos++] = c;
// example: newline means time to process buffer
// if this is set to 255 and master sends 255 then the whole things goes buck ape
if (c == 249)
process_it = true;
} // end of room available
} // end of interrupt routine SPI_STC_vect
// main loop - wait for flag set in interrupt routine
void loop (void)
{
int i;
/* SPI.begin();
SPI.transfer (SPDR);*/
if (process_it)
{
buf [pos] = 0;
Serial.print("\n\rstart...\n\r");
for(i=0; i< pos; i++){
Serial.print(buf[i], HEX); // print as an ASCII-encoded hexadecimal
Serial.print(" ");
if(buf[i] != i){
Serial.print("\n\rmismatch at byte no :");
Serial.print(i, DEC);
Serial.print("\n\r");
Serial.print("value is: ");
Serial.print(buf[i],DEC);
Serial.print("\n\r");
// do nothing as were now hosed
while(1) { }
}
}
Serial.print("\n\rend...\n\r");
pos = 0;
process_it = false;
} // end of flag set
} // end of loop
So yes, I am indeed dicking around with serial printing....however this works no problem at 1MHz and lower so I didn't think it was a problem. You think the serial printing might be slowing it down too much?
The serial printing needs almost a second in best case to print out the characters. During this time it interrupts the CPU after every character to put the next character in the buffer into the serial data register. The interrupts may block each other.
The other problem may be your interrupt routine. With 2MHz SPI speed you have 64 cycles between one SPI byte and the next one. A few cycles are needed to load the interrupt vector and save the processor state before entering your interrupt routine. Your routine is maybe 20-25 cycles so about half of the time slots available are used in the ISR leaving the other half to do the processing and printing to the serial port. At a speed of 9600 this may not be enough. Have you tried increasing your serial speed to let's say 57600? Does that change anything?
eriknyquist:
// SPI interrupt routine
ISR (SPI_STC_vect)
{
byte c = SPDR; // grab byte from SPI Data Register
// add to buffer if room
if (pos < sizeof buf)
{
buf [pos++] = c;
// example: newline means time to process buffer
// if this is set to 255 and master sends 255 then the whole things goes buck ape
if (c == 249)
process_it = true;
} // end of room available
} // end of int
[/QUOTE]Sometimes I can't resist an optimization opportunity when I see one. You are declaring a byte array of 300 bytes that will never overflow because pos is a byte. sizeof buf will never be reached. You'll never overwrite beyond the end of the buffer. You will have a limit of 256 bytes, but you will shave a few clock cycles. Since it wraparound to 256, do you need an overflow check? Overflow checks are important in many programs, but you never overflow because the byte becomes its own overflow preventer. Remove the overflow check for extra performance, if you've got a buffer bigger than the index variable, and if the buffer-full behavior isn't important.
This will perform slightly faster. It will have different behaviour when the buffer is full, it will start overwriting the beginning of the buffer again and suddenly discard the last 256 bytes of data (you'll have to test if this behavior is undesirable or not), but might work more reliably at a slightly higher speed. Since you're hosed anyway if the buffer is full, we probably don't care in what way it overflows? (depends on the application).
Here's a faster ISR which also is fixed-cycle (cycle count does not vary due to lack of "if" statement)
The only way I've managed to get really fast (can't remember how fast though) is to forget interrupts they are too slow, and use a tight polling loop. You ISR doesn't have a chance I think.
I'll see if I can find some old code that does that.
Thanks for all the suggestions guys, I really appreciate it. mrdrejohn, I tried your faster ISR and also with a serial speed of 115200. unfortunately same result.
tight polling loop- that's exactly what I need. I just don't know how to do it. I know I have to read the SPI status register (SPSR) which will tell me if data is available, I just don't know the correct way to implement it.
Anybody in the know?