Tool/software: Code Composer Studio
MSP430F5338: I setup the ADC12 with the following code:
//Allow ADC12_A to control Reference (as opposed to control via REF section)
REFCTL0 &= ~REFMSTR;
//Turn ON ADC12
ADC12CTL0 |= ADC12ON;
ADC12CTL1 |= (ADC12SSEL0|ADC12SSEL1); //Select SMCLK for ADC12_A Clock Source; no division
ADC12CTL1 |= ADC12SHP; //SAMPCON sampling signal sourced from sampling timer
//1K source impedance requires about 1.43 usec for settling to 12 bits
//This is about 20 ADC12CLK cycles
//Set it to 32 clock cycles
ADC12CTL0 |= (ADC12SHT00|ADC12SHT01); //Set sample time at 32 clock cycles
//Turn ON Internal Reference, set it at 2.5 V
ADC12CTL0 |= (ADC12REFON|ADC12REF2_5V);
//Set ADC12MEM0 conversion to use internal VREF+ as reference
ADC12MCTL0 |= ADC12SREF_1;
//Reference Buffer always-ON
ADC12CTL2 &= ~ADC12REFBURST;
//Configure P6.0 for ADC A0 Input
P6SEL |= BIT0;
//Select ADC12 A0 for input
ADC12MCTL0 |= ADC12INCH_0;
//Reset Flag
ADC12DataReady=0;
//Enable ADC12 A0 Interrupt
ADC12IE |= ADC12IE0;
//Enable Conversions (START via ADC12SC bit)
ADC12CTL0 |= ADC12ENC;
When I sample A0, there is a "dead zone" in the middle of the input range that gives a constant 2048 count output.
If I use AVCC as the REF+:
ADC12MCTL0 |= ADC12SREF_1;
the output is linear (and correct) through this region.
Any thoughts on why the internal REF+ is not working as the ADC12 reference?
