calculation in C for C2000 device - c

Question

I'm having some trouble with my C code. I have an ADC which will be used to determine whether to shut down (trip zone) the PWM which I'm using. But my calculation seem to not work as intended, because the ADC shuts down the PWM at the wrong voltage levels. I initiate my variables as:
float32 current = 5;
Uint16 shutdown = 0;
and then I calculate as:
// Save the ADC input to variable
adc_info->adc_result0 = AdcRegs.ADCRESULT0>>4; //bit shift 4 steps because adcresult0 is effectively a 12-bit register not 16-bit, ADCRESULT0 defined as Uint16
current = -3.462*((adc_info->adc_result0/1365) - 2.8);
// Evaluate if too high or too low
if(current > 9 || current < 1)
{
shutdown = 1;
}
else
{
shutdown = 0;
}
after which I use this if statement:
if(shutdown == 1)
{
EALLOW; // EALLOW protected register
EPwm1Regs.TZFRC.bit.OST = 1; // Force a one-shot trip-zone interrupt
EDIS; // end write to EALLOW protected register
}
So I want to trip the PWM if current is above 9 or below 1 which should coincide with an adc result of <273 (0x111) and >3428 (0xD64) respectively. The ADC values correspond to the voltages 0.2V and 2.51V respectively. The ADC measure with a 12-bit accuracy between the voltages 0 and 3V.
However, this is not the case. Instead, the trip zone goes off at approximately 1V and 2.97V. So what am I doing wrong?

Answer 1

adc_info->adc_result0/1365
Did you do integer division here while assuming float?
Try this fix:
adc_info->adc_result0/1365.0
Also, the #pmg's suggestion is good. Why spending cycles on calculating the voltage, when you can compare the ADC value immediately against the known bounds?
if (adc_info->adc_result0 < 273 || adc_info->adc_result0 > 3428)
{
shutdown = 1;
}
else
{
shutdown = 0;
}
If you don't want to hardcode the calculated bounds (which is totally understandable), then define them as calculated from values which you'd want to hardcode literally:
#define VOLTAGE_MIN 0.2
#define VOLTAGE_MAX 2.51
#define AREF 3.0
#define ADC_PER_VOLT (4096 / AREF)
#define ADC_MIN (VOLTAGE_MIN * ADC_PER_VOLT) /* 273 */
#define ADC_MAX (VOLTAGE_MAX * ADC_PER_VOLT) /* 3427 */
/* ... */
shutdown = (adcresult < ADC_MIN || adcresult > ADC_MAX) ? 1 : 0;
/* ... */
When you've made sure to grasp the integer division rules in C, consider a little addition to your code style: to always write constant coefficients and divisors with a decimal (to make sure they get a floating point type), e.g. 10.0 instead of 10 — unless you specifically mean integer division with truncation. Sometimes it may also be a good idea to specify float literal precision with appropriate suffix.