On Mon, 13 Jul 2015 13:04:05 -0700 (PDT), you wrote:
>
>Hey Everyone,
>
>I'm using the Beaglebone ADC on AIN0 (P9-39) to monitor a high voltage.
>I've used a divider (1/100) to put it within the 1.8V range of the ADC.I am
>using the cape overlay BB-ADC suggested by Derrick Molloy in Exploring BB.
>I've measured the voltage at the pin while using "cat
>/sys/devices/ocp.3/helper.12" and it agrees with my multimeter measurement
>at the pin. It seems the helper gives a direct mV value. They agree.
>Awesome!
>
>I'm trying to use this same technique with my C code, but for some reason
>I'm getting weird values from my read function. I think it has something to
>do with reading two bytes from the 12 bit ADC converter. I'm thinking I'm
>getting some junk on the end or the beginning, but masking and getting rid
>of the first or last 4 bits does not get the number I received from the
>cat'ing the helper device.
>
>Am I reading this wrong with my code? Does anyone have any ideas to get
>the correct 12 bits?
Several generic things may be bothering this:
1) you have interrupts enabled and you're getting a task switch during
the read. Solution: Make the read atomic.
2) the register order can be wrong. Some processors require a
specific order to read 8 bit registers. If this processor has that
same problem and you're not reading the entire register at one gulp,
then that can be a problem. This should be compensated for in the
compiler, though, so interrupts enabled could be more of a problem.
Harvey
>
>Thanks! I think I gave enough information below.
>
>*Test Code:*
>
>#include <stdio.h>
>#include <stdlib.h>
>#include <string.h>
>#include <stdint.h>
>#include <errno.h>
>#include <unistd.h>
>#include <fcntl.h>
>#include <poll.h>
>#include <sys/ioctl.h>
>#include <linux/spi/spidev.h>
>#include "DDC2.h"
>
>
>int main ( int argc, const char *argv[] )
>{
>float voltage;
>voltage = atof ( argv[1] );
>set_high_voltage ( voltage );
>read_raw_high_voltage ( );
>return 0;
>}
>
>*Read Function:*
>
>/***********************************************************************
>* Function- read_high_voltage
>* Description- Reads the ADC on AIN0 and returns the measured value of the
>* high voltage.
>************************************************************************/
>int read_raw_high_voltage(void)
>{
>uint16_t fd;
>uint16_t value;
>uint16_t value1;
>uint16_t value2;
>//Open "analog0=/sys/devices/ocp.3/helper.12/AIN0"
>if ( ( fd = open ( analog0, O_RDONLY) ) < 0 )
> {
> perror ("SPI: Can't open device.");
> return -1;
> }
> read ( fd, &value, 2);
> value1=value & 0x0FFF;
> value2=value>>4;
> printf ("All Bits=%d\n", value);
> printf ("Bits 0-12=%d\n", value1);
> printf ("Bits 4-16=%d\n", value1);
>return(0);
>}
>
>*From Command line:*
>
>./test 60
>
>*Output:*
>
>Voltage Given=60.0
>All Bits= 14389
>Bit 0-12=2101
>Bit 4-16=899
>
>Measured Voltage from Multimeter= 600mV (measured after 1/100 voltage
>divider)
>
>Cat from the helper device = 587
--
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