need help!!!  arduino SPI programming the MCP6S21

Forum 2005-2010 (read only) Hardware Interfacing
1

Hi,
Can somebody please help me understand how to program using the SPI library on MCP6S21. I'm using the Arduino Duemilanove(Atmega328) with the software ver 0019.
The MCP6S21 is 8-PDIP from microchip programmable gain amplifier (PGA) using SPI: SCK, SI, CS (only 3 pins - input only).
I've tried over and over again to simply write a program that changes the amplifier gain to X2 but with no success at all. The MCP6S21 basically programmed through the SPI interface using 16-bit words. link to DATASHEET: http://ww1.microchip.com/downloads/en/DeviceDoc/21117a.pdf

From what I see in the datasheet it supposed to be very simple but I've tried many variations of programs like the arduino examples: DigitalPotControl, BarometricPressureSensor or even something more close: Arduino Playground - MCP3208
so far nothing worked.

I'm new to SPI and if someone can shade a light for me I'll be very grateful...

2

Best if you post some simple code of what you have tried and we can see if there is anything that we can spot that is wrong.
I would advise pull up resistors of 4K7 on both the data and clock and remember that the I2C is on analogue pins 4 & 5 and not the digital ones. (and on the mega they are different again)

3

Here is my current program:

#include <SPI.h>
/*

  • CS - to digital pin 10 (SS pin)
  • SDI - to digital pin 11 (MOSI pin)
  • CLK - to digital pin 13 (SCK pin)
    */

#define CS 10

void setup() {
// set CS as an output:
pinMode (CS, OUTPUT);
// initialize SPI:
SPI.begin();
SPI.setBitOrder(MSBFIRST); // MSB first bit order
SPI.setDataMode(SPI_MODE0); //SPI Mode 0
SPI.setClockDivider(SPI_CLOCK_DIV4); // 16MHZ/4 = 4MHZ
// take the SS pin low to select the chip:
digitalWrite(CS,LOW);
// send in the address and value via SPI:
SPI.transfer(B01000000); // Instruction Register set to "write to register"
SPI.transfer(B00000001); // Gain Register set to "Gain of +2"
// take the SS pin high to de-select the chip:
digitalWrite(CS,HIGH);
SPI.end();
}

void loop() {
}

4

@Grumpy: its SPI not I2C
@Hersh: You are way too fast, reduce to 1 MHz or slower
In principle the DigPot example

comes closest and should work; of course choosing other pay load bytes.

Maybe you should set CS to high in the beginning, as it could confuse him if clock is not yet low... but I really do not think that is an issue..

5

deSilva:
First thanks for your post,

I've tried what u suggested:
SPI.setClockDivider(SPI_CLOCK_DIV16);
or
SPI.setClockDivider(SPI_CLOCK_DIV32);
even
SPI.setClockDivider(SPI_CLOCK_DIV64);
or changing CS to high in the beginning,
but nothing changed a thing . I am still reading on the output wire the same voltage in the CH0 (input) , meaning x1 gain and not x2.
maybe there is a problem with the SPI library ?

Here is my circuit schematic:

6

Vref should not be set to ground

7

deSilva:

Based on what u say VREF should not be set to ground?
according to the datasheet of the MCP6S21:

for VREF=VSS=GND: VOUT=GAINCH0
and for VDD>=VREF>VSS=GND: VOUT=VREF+GAIN(CH0-VREF)

8

I've found out what was wrong: the problem was in my circuit..
I used VSS<0 and I missed that it must be: |VDD-VSS|<7v
as mention in the datasheet...
Now it's working great and it's a very precise gain amp.