RFM69HCW 915 MHz radio module: Sending a stream of int values rather than char[ ]

Question

I'm trying to send a stream of int values (0-4096) from one arduino to another using an RFM69HCW module.

Here's my main code:

#include <RFM69.h>
#include <SPI.h>
#define NETWORKID     0   // Must be the same for all nodes
#define MYNODEID      1   // My node ID
#define TONODEID      2   // Destination node ID
#define FREQUENCY     RF69_915MHZ
#define ENCRYPT       true // Set to "true" to use encryption
#define ENCRYPTKEY    "TOPSECRETPASSWRD" // Use the same 16-byte key on all nodes
#define RFM69_CS      10
#define RFM69_IRQ     2
#define RFM69_IRQN    0  // Pin 2 is IRQ 0!
#define RFM69_RST     9
#define IS_RFM69HCW   true
#define USEACK        true // Request ACKs or not
#define LED           9 // LED positive pin
#define GND           8 // LED ground pin
#define PULSEPIN      0 // Heart Rate Monitor at pin A0

RFM69 radio = RFM69(RFM69_CS, RFM69_IRQ, IS_RFM69HCW, RFM69_IRQN);
String pulsearray;
bool recHR = true;
unsigned long oldtime = 0;
int interval = 5000;
static int readtime = 3000;
static int sampletime = 10; // 1000/this is the thing
static int txinterval = 1000; // delay between transmissions

void setup() {
  initSerial();
  initLed();
  initRadio();
}

void loop() {
  unsigned long newtime = millis();
  if (!recHR && newtime - oldtime >= interval) {
    recHR = true;
    oldtime = newtime;
    Serial.println("recording HR now");
  }

  while (recHR) {
    unsigned long newtime = millis();
    if (newtime - oldtime >= sampletime) {
      readhr();
    }
    if (newtime - oldtime >= readtime) {
      recHR = false;
      oldtime = newtime;
      Serial.println("stopped reading HR");
    }
  }
}

And the function I'm using:

void readhr() {
  for (int i = 0; i < 20; i++) {
    int pulse = analogRead(PULSEPIN);
    pulsearray = pulsearray + pulse;
    pulsearray = pulsearray + '\n';
    Serial.println(pulse);
  }
}

The library function sendWithRetry() requires the following arguments:

bool RFM69::sendWithRetry(uint8_t toAddress, const void* buffer, uint8_t bufferSize, uint8_t retries, uint8_t retryWaitTime)

I would like a way to set up a buffer which can be transmitted in multiple packets during the 5 second analogRead() downtime. Using a string as my buffer doesn't work when I try to transmit the first n (<60) characters in the String. I've also tried using a char[ ] as my buffer (a lot more troublesome when storing int values) but it still gives the same error:

exit status 1
no matching function for call to 'RFM69::sendWithRetry(int, String&, unsigned int)'

Answer 1

You need to send the address of a buffer and the size, in bytes, of that buffer. What that buffer consists of is neither here nor there.

If you have an array of 10 ints you have a 20 byte buffer (since one int is 2 bytes on an 8-bit Arduino).

So you just pass it the address of the buffer (which as an array it is already just an address) and the size of that buffer:

int values[10];
// whatever to fill the array
RFM69::sendWithRetry(20, values, 20, 4, 1000);

As long as the array is statically created (e.g., a global variable with a fixed number of entries) you can use sizeof() to get the size in bytes:

RFM69::sendWithRetry(20, values, sizeof(values), 4, 1000);

As long as you receive at the other and using the same type of buffer it should be completely transparent.

To keep the system cross-platform and able to run on an Arduino that is either 8 or 32 bits and have them completely interoperable it is best to use variables with a fixed size. Instead of int use int16_t which is always 2 bytes and signed on every platform everywhere:

int16_t values[10];

Or use uint16_t if you want it unsigned.