Electronic – Problem with while loop

avrcmicrocontrollerprogramming

I am trying to read external ADC AD7798 values using ATmega32-A controller. In the datasheet, Status register Bit 7 (SR7) indicates the conversion is finished or not:

Ready Bit. Cleared when data is written to the data register. Set
after the data register is read or after a period of time before the
data register is updated with a new conversion result to indicate to
the user not to read the conversion data. It is also set when the part
is placed in power-down mode. The end of a conversion is indicated by
the DOUT/RDY pin. This pin can be used as an alternative to the status
register for monitoring the ADC for conversion data.

Now I want to write code to check if the Status register MSB (Bit7) is 0 or not. So when it is 0, only then can I issue a read command.

I have written code like this:

unsigned char CheckStatus(void)
{
            char adcStatus; 
            spi(0x40);
            adcStatus = spi(0xFF);
            while((adcStatus & 0x80)!=0x80);                         
            return adcStatus;
}

But it is not working.

My code explanation:

Issuing the read command to read ADC status register.
Reading ADC status register value and stored in adcStatus variable.
Checking the MSB bit is not equal to 1. (I am not sure if this while loop is correct or not.)

I want to check if the Status register MSB is not equal to 1 so that I can issue the read command (0x58) to read in the ADC values. If the status register MSB is equal to 1 then I can't read ADC values.

What am I doing wrong?

Best Answer

You need to re-read the ADC status register inside the loop; otherwise, you're just re-testing the bit you read the first time. Also, you need to reverse the sense of the test — you want to repeat the test if the bit is "1" and drop out of the loop once it switches to "0". It may also be necessary to toggle chip select to the ADC on each read, as shown in the comments below.

unsigned char CheckStatus(void)
{
  unsigned char adcStatus;
  do {
    /* TBD: assert chip select here? */
    spi(0x40);
    adcStatus = spi(0xFF);
    /* TBD: negate chip select here? */
  } while ((adcStatus & 0x80) == 0x80);                         
  return adcStatus;
}

Related Solutions

Electronic – ADC registers setup using spi communication

While this datasheet is making my eyes bleed with the complexity of doing a simple conversion/read, you need to make some simple changes.

I'm not sure why people are making this harder than it really is. You pull the chip select line low, and the target IC should simply read whatever you give it until you pull it high again. Just because your method takes an 8-bit value doesn't mean you can't call it twice to pass a 16-bit value to your IC. There is no need to bit-bang anything. That is nonsense.

Call your SPI method to tell the ADC you're looking to write to the configuration register:

spi(0x10);

As I understand from the datasheet, you can immediately write your 16-bit value to the configuration register after that, so you'd do:

spi(0x07);
spi(0x10);

I forget which way that'll end up being assembled on the target IC side, so you could simply reverse them if things don't work right. There's no need to bit shift any values at all. Worst case scenario is you have to pull the CS line back high before pulling it low again to actually send the data to write to the configuration register.

Otherwise, this is super simple. Remember, if the target SPI device expects more than one byte, i.e. a 16-bit value, chances are good that sending the bytes in the order they'd otherwise appear normally (so if you want to send 0x1234, you'd have 0x12 and 0x34) will work fine and you won't need to shift anything.

Electronic – bit banging

Bit banging is creating the whole series of pulses in software, instead of relying on a piece of hardware inside the microcontroller.

Many microcontrollers have a hardware SPI, and then all you have to do is write a byte to the output register, and the SPI controller will shift the data out, and at the same time receive data from the slave. You can get an interrupt when the transfer is complete, and then read the received data.

But some microcontrollers don't have the SPI hardware on board and then you have to simulate it by doing everything manually. SPI has a number of different modes, I'll use this pulse diagram as an example:

enter image description here

So while a dedicated SPI controller takes care of all the pulses, data shifting and timing, when bit-banging you have to take every action yourself:

Make Slave Select low  
Short delay
Do 8 times
  Make the SCK (Serial Clock) pin low 
  Make the MOSI (Master-Out-Slave-In) pin high or low depending on bit 7 of the data  
  Add brief delay  
  Make the SCK output high
  Read MISO (Master-In-Slave-Out) pin
  Shift received data left, and shift the bit just read in as bit 0   
  Add brief delay  
  Shift the data byte 1 bit left
Make Slave Select high again

Bit-banging SPI is relatively simple, the code for bit-banging I2C for instance will be more complex, and you'll need a timer somehow if you want to bit-bang the UART protocol.

Best Answer

Related Solutions

Electronic – ADC registers setup using spi communication

Electronic – bit banging

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