Look I've read the data sheet, several times.
It clearly states in the datasheet that the output code for Vref should be 111111111111111111111111, which converts to 16777215. I'm asking why I'm not getting the full resolution and why the output is not going all the way up to 16777215 but only half that.
I'm not the sort of person that doesn't read up on things before they ask, I've spend literally several hours trying to find information on this and to work it out but I don't get it.
Telling me to read the datasheet again is just really patronising after how much effort I've put into trying to work this out for myself.
My code's here:
/*
Interface between Arduino DM board and Linear Tech LTC2440 24-bit ADC
Nov. 12 2010 John Beale
LTC2440 <----------> Arduino
11: /CS <- to digital pin 10 (SS pin)
7: MOSI <- to digital pin 11 (MOSI pin)
12: MISO -> to digital pin 12 (MISO pin)
13: SCLK <- to digital pin 13 (SCK pin)
10: /EXT - ground
1: GND - ground
2: VDD3 - 5V supply
*/
#include <SPI.h> // include the SPI library
// set I/O pins used in addition to clock, data in, data out
const byte slaveSelectPin = 10; // digital pin 10 for /CS
const byte resetPin = 9; // digital pin 9 for /RESET
const int nsamples = 1; // how many ADC readings to average together
// SPI_CLOCK_DIV16 gives me a 1.0 MHz SPI clock, with 16 MHz crystal on Arduino
void setup() {
Serial.begin(9600); // set up serial comm to PC at this baud rate
pinMode (slaveSelectPin, OUTPUT);
pinMode (resetPin, OUTPUT);
digitalWrite(slaveSelectPin,HIGH); // chip select is active low
digitalWrite(resetPin,HIGH); // reset is active low
SPI.begin(); // initialize SPI, covering MOSI,MISO,SCK signals
SPI.setBitOrder(MSBFIRST); // data is clocked in MSB first
SPI.setDataMode(SPI_MODE0); // SCLK idle low (CPOL=0), MOSI read on rising edge (CPHI=0)
SPI.setClockDivider(SPI_CLOCK_DIV16); // system clock = 16 MHz, chip max = 1 MHz
}
// =============================================================================
// Main Loop:
// acquire 'nsamples' readings, convert to units of volts, and send out on serial port
void loop() {
int i;
long secs;
float mins;
double volts;
long in; // incoming serial 32-bit word
long sum = 0;
for (i=0; i<nsamples; i++) {
in = SpiRead();
in &= 0x1FFFFFFF; // force high three bits to zero
in = in>>5; // truncate lowest 5 bits
sum += in;
delay(198); // (msec). Total Looptime: +2 msec (overhead for comms)
}
// volts = in * 2.5 / 8.388607; // 0x7fffff = 8388607
volts = sum * (2.048/8388607); // microvolts
volts = volts / nsamples;
Serial.println(volts, 6);
} // end main loop
// =================================================================
// SpiRead() -- read out 4 bytes from LTC2440 chip via SPI interface
// =================================================================
long SpiRead(void) {
long result = 0;
long b;
// long result2 = 0;// MOSI/SDI pin 7 HIGH => 7 Hz, best resolution
digitalWrite(slaveSelectPin,LOW); // take the SS pin low to select the chip
delayMicroseconds(1); // probably not needed, only need 25 nsec delay
b = SPI.transfer(0xff); // B3
result = b<<8;
b = SPI.transfer(0xff); // B2
result |= b;
result = result<<8;
b = SPI.transfer(0xff); // B1
result |= b;
result = result<<8;
b = SPI.transfer(0xff); // B0
result |= b;
// take the SS pin high to de-select the chip:
digitalWrite(slaveSelectPin,HIGH);
return(result);
}
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