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I have a DAC8811 that has been modified for +-10V operation and it is connected to a F28335. I have been testing to see what is going on with my output voltage with respect to the data signal being sent to it. I have noticed that the code 0xFFFF will pass through the Vref (10V) to the first amplifier on the board. If 0xFFFE the amplifier will output 5 volts. Which seems like odd behavior that I haven't seen documented.
When I increment the value being sent to the DAC by odd values it starts sending the voltage to rails while also producing the saw tooth waveform. If it is increment by a even value it behaves as expected. If I increment the value 10 or greater it does not have this behavior. It really can be seen when I single step through the code. Send one code and see what voltage it goes to and continue that.
These scope traces have the output updated at about a 5kHz rate and the SPI clock is at 40MHz. Here is a picture of the output with a odd increment:
With an even increment and what is expected:
Below is the SPI code being used to setup the transfer. I have tried various SPI clocks to see if that might be one of the issues, but it does not change what is going on.
void spi_init()
{
SpiaRegs.SPICCR.all =0x000F; // Reset on, rising edge, 16-bit char bits
SpiaRegs.SPICTL.all =0x0006; // Enable master mode, normal phase,
// enable talk, and SPI int disabled.
SpiaRegs.SPIBRR =0x0008;
//SpiaRegs.SPIBRR =0x00A7;
SpiaRegs.SPICCR.all =0x009F; // Relinquish SPI from Reset
SpiaRegs.SPIPRI.bit.FREE = 1; // Set so breakpoints don't disturb xmission
}
void spi_xmit(Uint16 a)
{
SpiaRegs.SPITXBUF=a;
}
void spi_fifo_init()
{
// Initialize SPI FIFO registers
SpiaRegs.SPIFFTX.all=0xE040;
SpiaRegs.SPIFFRX.all=0x204f;
SpiaRegs.SPIFFCT.all=0x0;
}
Thank you Bryan! For some reason I finally got access. Here is the attachment of the oscilloscope result. The transitions on voltage would be when values are being sent to the DAC.
Here is the CS, Data line, and Clock:
Here is the Vout line and the Main output with an odd increment:
Here is 0x0101 being sent to the DAC:
When 0x0100 being sent to the DAC:
The capture of 0x0100 should be the way it should react. As it can be seen giving a 0x0101 the DAC output goes to -5V. Which is the rail of the voltage. All odd values have been doing this. If I send 0x1001 it does the same thing. Etc Etc.
