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cap.c
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/* ************************************************************************
*
* capacitor measurements
*
* (c) 2012-2018 by Markus Reschke
* based on code from Markus Frejek and Karl-Heinz Kübbeler
*
* ************************************************************************ */
/*
* local constants
*/
/* source management */
#define CAP_C
/*
* include header files
*/
/* local includes */
#include "config.h" /* global configuration */
#include "common.h" /* common header file */
#include "variables.h" /* global variables */
#include "functions.h" /* external functions */
/* ************************************************************************
* ESR measurement
* ************************************************************************ */
#ifdef SW_ESR
/*
* set up timer for delay
* - uses Timer0 as MCU cycle timer
*
* requires:
* - number of MCU cycles
*
* returns:
* - 1 on success
* - 0 on error
*/
uint8_t SetUpDelayTimer(uint8_t Cycles)
{
uint8_t Flag = 0; /* return value */
/*
* adjust cycles for DelayTimer():
* - -4 for function call
* - -3 for starting timer
* - -2 for waiting loop
* - -4 for stopping timer
* - -4 for return
* - total: 17
*/
#define CYCLE_OFFSET 17
if (Cycles < CYCLE_OFFSET) return Flag; /* check for required minimum */
Cycles -= CYCLE_OFFSET; /* substract cycles for DelayTimer() */
#undef CYCLE_OFFSET
/*
* set up Timer0:
* - CTC mode (count up to OCR0A)
* - prescaler 1 to match MCU cycles
*/
TCCR0B = 0; /* stop timer */
TCCR0A = (1 << WGM01); /* set CTC mode, disable output compare pins */
OCR0A = Cycles; /* set number of MCU cycles */
/* todo: check if we have to substract one cycle for setting the flag */
Flag = 1; /* signal success */
return Flag;
}
/*
* start timer and wait for timeout
* (requires prior call of SetUpPulseTimer() for setup)
*/
void DelayTimer(void)
{
TCNT0 = 0; /* reset counter to 0 */
TCCR0B = (1 << CS00); /* start timer by setting prescaler */
while (!(TIFR0 & (1 << OCF0A))); /* wait for output compare A match flag */
TCCR0B = 0; /* stop timer */
TIFR0 = (1 << OCF0A); /* clear flag */
}
/*
* measure ESR
*
* requires:
* - pointer to cap data structure
*
* returns:
* - ESR in 0.01 Ohm
* - UINT16_MAX on any problem
*/
uint16_t MeasureESR(Capacitor_Type *Cap)
{
uint16_t ESR = UINT16_MAX; /* return value */
uint16_t U_1; /* voltage at probe 1 with pos. pulse unloaded */
uint16_t U_2; /* voltage at probe 2 with pos. pulse loaded */
uint16_t U_3; /* voltage at probe 2 with neg. pulse unloaded */
uint16_t U_4; /* voltage at probe 1 with neg. pulse loaded */
uint8_t Probe1; /* probe #1 */
uint8_t Probe2; /* probe #2 */
uint8_t ADC_Mask; /* bit mask for ADC */
uint8_t n; /* counter */
uint32_t Sum_1; /* sum #1 */
uint32_t Sum_2; /* sum #2 */
uint32_t Value;
/* check for a capacitor >= 10nF */
if ((Cap == NULL) ||
(CmpValue(Cap->Value, Cap->Scale, 10, -9) < 0)) return ESR;
/*
* Hint:
* - When we would use MilliSleep here we'd have to change the MCU
* sleep mode to idle to keep the timer running in sleep mode.
*/
/*
* init stuff
*/
DischargeProbes(); /* try to discharge probes */
if (Check.Found == COMP_ERROR) return ESR; /* skip on error */
UpdateProbes(Cap->A, Cap->B, 0); /* update probes */
Probe1 = Probes.ADC_1; /* ADC MUX for probe-1 */
Probe2 = Probes.ADC_2; /* ADC MUX for probe-2 */
Probe1 |= ADC_REF_BANDGAP; /* select bandgap reference */
Probe2 |= ADC_REF_BANDGAP; /* select bandgap reference */
/* bitmask to enable and start ADC */
ADC_Mask = (1 << ADSC) | (1 << ADEN) | (1 << ADIF) | ADC_CLOCK_DIV;
/* init variables */
Sum_1 = 1; /* 1 to prevent division by zero */
Sum_2 = 1; /* 1 to prevent division by zero */
/*
* We have to create a delay to shift the middle of the current pulse to
* the ADC's S&H. S&H happens at 1.5 ADC clock cycles after starting the
* conversion. We synchronize to a dummy conversion done directly before,
* so we have 2.5 ADC clock cycles to S&H. The required delay between the
* dummy conversion and S&H of the next conversion is:
* MCU cycles for 2.5 ADC clock cycles
* - MCU cycles for waiting loop for completion of dummy conversion (4)
* - MCU cycles for starting next conversion (2)
* - MCU cycles for fixed pre-delay of 10µs
* - MCU cycles for enabling pulse (4)
* - MCU cycles for half-pulse (2µs)
*
* MCU ADC MCU cycles for
* clock clock 2.5 ADC cycles pre-delay half-pulse delay
* ---------------------------------------------------------------
* 8 MHz 125 kHz 160 (20µs) 80 (10µs) 16 (2µs) 54
* 32 (4µs) 38
* 16 MHz 125 kHz 320 (20µs) 160 (10µs) 32 (2µs) 118
* 64 (4µs) 86
* 20 MHz 156.25 320 (16µs) 200 (10µs) 40 (2µs) 70
* 80 (4µs) 30
*
* Skipping the second half-pulse allows us to measure low value caps too.
*/
/* delay for pulse */
/* MCU cycles for one ADC cycle * 2.5 - MCU cycles for 10µs
- MCU cycles for half-pulse - 10 */
U_1 = ((MCU_CYCLES_PER_ADC * 25) / 10) - (MCU_CYCLES_PER_US * 10)
- (MCU_CYCLES_PER_US * 2) - 10;
#if CPU_FREQ == 8000000
/* magic time shift to compensate missing second half-pulse */
U_1 -= 4;
#endif
n = (uint8_t)U_1;
/* set up delay timer */
if (SetUpDelayTimer(n) == 0) return ESR; /* skip on error */
/*
* measurement loop:
* - simulate AC by short positive and negative pulses
* - measure start voltage without DUT
* - measure pulse voltage with DUT
* - pre-charge DUT with a negative pulse of half length to
* to compensate voltage rise by positive charging pulse
* - 16 & 20 MHz MCUs seem to measure higher ESR values
*/
ADC_PORT = 0; /* set ADC port to low */
ADMUX = Probe1; /* set input channel to probe-1 & set bandgap ref */
wait10ms(); /* time for voltage stabilization */
U_2 = 50; /* don't start with positive half-pulse */
U_4 = 0; /* start with a negative half-pulse */
n = 255; /* set loop counter */
while (n > 0)
{
wdt_reset(); /* reset watchdog */
/*
* mitigate runaway of cap's charge/voltage
*/
if (U_4 < 50)
{
/* charge cap a little bit more (negative pulse) */
/* set probes: GND -- probe-2 / probe-1 -- Rl -- 5V */
ADC_DDR = Probes.Pin_2; /* pull down probe-2 directly */
R_PORT = Probes.Rl_1; /* pull up probe-1 via Rl */
R_DDR = Probes.Rl_1; /* enable pull up */
wait2us(); /* wait half-pulse */
R_DDR = 0; /* disable any pull up */
R_PORT = 0; /* reset probe resistors */
}
if (U_2 < 50)
{
/* charge cap a little bit more (positive pulse) */
/* set probes: GND -- probe-1 / probe-2 -- Rl -- 5V */
ADC_DDR = Probes.Pin_1; /* pull down probe-1 directly */
R_PORT = Probes.Rl_2; /* pull up probe-2 via Rl */
R_DDR = Probes.Rl_2; /* enable pull up */
wait2us(); /* wait half-pulse */
R_DDR = 0; /* disable any pull up */
R_PORT = 0; /* reset probe resistors */
}
/*
* forward mode, probe-1 only (probe-2 in HiZ mode)
* set probes: GND -- probe-1 -- Rl -- 5V / probe-2 -- HiZ
* get voltage at probe-1 (voltage at RiL)
*/
ADC_DDR = Probes.Pin_1; /* pull down probe-1 directly to GND */
R_PORT = Probes.Rl_1; /* pull up probe-1 via Rl */
R_DDR = Probes.Rl_1; /* enable resistor */
ADMUX = Probe1; /* set input channel to probe-1 & set bandgap ref */
/* run dummy conversion for ADMUX change */
ADCSRA = ADC_Mask; /* start conversion */
while (ADCSRA & (1 << ADSC)); /* wait until conversion is done */
/* real conversion */
ADCSRA = ADC_Mask; /* start conversion */
while (ADCSRA & (1 << ADSC)); /* wait until conversion is done */
U_1 = ADCW; /* save ADC value */
/*
* forward mode, positive charging pulse
* set probes: GND -- probe-1 / probe-2 -- Rl -- 5V
* get voltage at probe-2 (voltage at DUT, i.e. RiL + ESR)
*/
ADMUX = Probe2; /* set input channel to probe-2 & set bandgap ref */
/* run dummy conversion for ADMUX change */
ADCSRA = ADC_Mask; /* start conversion */
while (ADCSRA & (1 << ADSC)); /* wait until conversion is done */
/* read ADC in the mid of a positive charging pulse */
ADCSRA = ADC_Mask; /* start conversion with next ADC clock cycle */
wait10us(); /* fixed pre-delay */
DelayTimer(); /* delay for pulse */
R_PORT = Probes.Rl_2; /* pull up probe-2 via Rl */
R_DDR = Probes.Rl_2; /* enable resistor */
wait2us(); /* first half-pulse */
/* S/H happens here */
#if CPU_FREQ < 8000000
wait2us(); /* second half-pulse */
#endif
R_PORT = 0; /* set resistor port to low */
R_DDR = 0; /* set resistor port to HiZ */
while (ADCSRA & (1 << ADSC)); /* wait until conversion is done */
U_2 = ADCW; /* save ADC value */
/*
* reverse mode, probe-2 only (probe-1 in HiZ mode)
* set probes: GND -- probe-2 -- Rl -- 5V / probe-1 -- HiZ
* get voltage at probe 2 (voltage at RiL)
*/
ADC_DDR = Probes.Pin_2; /* pull down probe-2 directly */
R_PORT = Probes.Rl_2; /* pull up probe-2 via Rl */
R_DDR = Probes.Rl_2; /* enable resistor */
ADMUX = Probe2; /* set input channel to probe-2 & set bandgap ref */
/* run dummy conversion for ADMUX change */
ADCSRA = ADC_Mask; /* start conversion */
while (ADCSRA & (1 << ADSC)); /* wait until conversion is done */
/* real conversion */
ADCSRA = ADC_Mask; /* start conversion */
while (ADCSRA & (1 << ADSC)); /* wait until conversion is done */
U_3 = ADCW; /* save ADC value */
/*
* reverse mode, negative charging pulse
* set probes: GND -- probe-2 / probe-1 -- Rl -- 5V
* get voltage at probe-1 (voltage at DUT, i.e. RiL + ESR)
*/
ADMUX = Probe1; /* set input channel to probe-1 & set bandgap ref */
/* run dummy conversion for ADMUX change */
ADCSRA = ADC_Mask; /* start conversion */
while (ADCSRA & (1 << ADSC)); /* wait until conversion is done */
/* read ADC in the mid of a negatve charging pulse */
ADCSRA = ADC_Mask; /* start conversion with next ADC clock cycle */
wait10us(); /* fixed pre-delay */
DelayTimer(); /* delay for pulse */
R_PORT = Probes.Rl_1; /* pull up probe-1 via Rl */
R_DDR = Probes.Rl_1; /* enable resistor */
wait2us(); /* first half-pulse */
/* S/H happens here */
#if CPU_FREQ < 8000000
wait2us(); /* second half-pulse */
#endif
R_PORT = 0; /* set resistor port to low */
R_DDR = 0; /* set resistor port to HiZ */
while (ADCSRA & (1 << ADSC)); /* wait until conversion is done */
U_4 = ADCW; /* save ADC value */
/*
* manage measured values
*/
Sum_1 += U_1; /* positive pulse without DUT */
Sum_1 += U_3; /* negative pulse without DUT */
Sum_2 += U_2; /* positive pulse with DUT */
Sum_2 += U_4; /* negative pulse with DUT */
n--; /* next loop run */
}
/*
* process measurements
*/
if (Sum_2 > Sum_1) /* valid measurement */
{
/*
* calculate ESR
* - ESR = U_ESR / I_ESR
* with U_ESR = (U2 or U4) and I_ESR = (U1 or U3) / RiL
* ESR = (U2 or U4) * RiL / (U1 or U3)
* - since we divide (U2 or U4) by (U1 or U3), we don't need to convert
* the ADC value into a voltage and simply desample the sums.
* - so ESR = Sum_2 * RiL / Sum_1
* - for a resolution of 0.01 Ohms we have to scale RiL to 0.01 Ohms
*/
/* voltage across the DUT (raw value) */
Sum_2 -= Sum_1; /* subtract voltage at DUT's low side (RiL) */
/* ESR = Sum_2 * RiL / Sum_1 */
Value = (uint32_t)(NV.RiL * 10); /* RiL in 0.01 Ohms */
Value *= Sum_2; /* sum of raw values for voltage across DUT */
Value /= Sum_1; /* sum of raw values for voltage at RiL */
U_1 = (uint16_t)Value;
/* consider probe resistance */
if (U_1 > NV.RZero) /* larger than offset */
{
U_1 -= NV.RZero; /* subtract offset */
ESR = U_1; /* got result */
}
else /* offset problem or zero */
{
/* should only happen for large caps */
if (CmpValue(Cap->Value, Cap->Scale, 1000, -6) > 0)
{
ESR = 0; /* can't be less than 0 Ohms */
}
}
}
/* update Uref flag for next ADC run */
Cfg.RefFlag = ADC_REF_BANDGAP; /* update flag */
return ESR;
}
#endif
#ifdef SW_OLD_ESR
/*
* set up Timer0 as MCU cycle timer
*
* requires:
* - number of MCU cycles
*
* returns:
* - 1 on success
* - 0 on error
*/
uint8_t SetupDelayTimer(uint8_t Cycles)
{
uint8_t Flag = 0; /* return value */
/*
* adjust cycles for DelayTimer():
* - -4 for function call
* - -3 for starting timer
* - -2 for waiting loop
* - -4 for stopping timer
* - -4 for return
* - total: 17
*/
#define CYCLE_OFFSET 17
if (Cycles < CYCLE_OFFSET) return Flag; /* check for required minimum */
Cycles -= CYCLE_OFFSET; /* substract cycles for DelayTimer() */
#undef CYCLE_OFFSET
/*
* set up timer0:
* - CTC mode (count up to OCR0A)
* - prescaler 1 to match MCU cycles
*/
TCCR0B = 0; /* stop timer */
TCCR0A = (1 << WGM01); /* set CTC mode, disable output compare pins */
OCR0A = Cycles; /* set number of MCU cycles */
/* todo: check if we have to substract one cycle for setting the flag */
Flag = 1; /* signal success */
return Flag;
}
/*
* start timer and wait for timeout
* (requires prior call of SetupDelayTimer() for setup)
*/
void DelayTimer(void)
{
TCNT0 = 0; /* reset counter to 0 */
TCCR0B = (1 << CS00); /* start timer by setting prescaler */
while (!(TIFR0 & (1 << OCF0A))); /* wait for output compare A match flag */
TCCR0B = 0; /* stop timer */
TIFR0 = (1 << OCF0A); /* clear flag */
}
/*
* measure ESR
*
* returns:
* - ESR in 0.01 Ohm
* - UINT16_MAX on any problem
*/
uint16_t MeasureESR(Capacitor_Type *Cap)
{
uint16_t ESR = UINT16_MAX; /* return value */
uint16_t U_1; /* voltage at probe 1 with pos. pulse unloaded */
uint16_t U_2; /* voltage at probe 2 with pos. pulse loaded */
uint16_t U_3; /* voltage at probe 2 with neg. pulse unloaded */
uint16_t U_4; /* voltage at probe 1 with neg. pulse loaded */
uint8_t Probe1; /* probe #1 */
uint8_t Probe2; /* probe #2 */
uint8_t ADC_Mask; /* bit mask for ADC */
uint8_t n; /* counter */
uint32_t Sum_1; /* sum #1 */
uint32_t Sum_2; /* sum #2 */
uint32_t Value;
/* check for a capacitor >= 0.18µF */
if ((Cap == NULL) ||
(CmpValue(Cap->Value, Cap->Scale, 180, -9) < 0)) return ESR;
/*
* init stuff
*/
DischargeProbes(); /* try to discharge probes */
if (Check.Found == COMP_ERROR) return ESR; /* skip on error */
UpdateProbes(Cap->A, Cap->B, 0); /* update probes */
Probe1 = Probes.ADC_1; /* ADC MUX for probe-1 */
Probe2 = Probes.ADC_2; /* ADC MUX for probe-2 */
Probe1 |= ADC_REF_BANDGAP; /* select bandgap reference */
Probe2 |= ADC_REF_BANDGAP; /* select bandgap reference */
/* bitmask to enable and start ADC */
ADC_Mask = (1 << ADSC) | (1 << ADEN) | (1 << ADIF) | ADC_CLOCK_DIV;
/* init variables */
Sum_1 = 1; /* 1 to prevent division by zero */
Sum_2 = 1; /* 1 to prevent division by zero */
/*
* We have to create a delay to shift the middle of the pulse to the ADC's
* S&H. S&H happens at 1.5 ADC clock cycles after starting the conversion.
* We synchronize to a dummy conversion done directly before, so we have
* 2.5 ADC clock cycles to S&H. The time between the completed dummy
* conversion and S&H of the next conversion is:
* 2.5 ADC clock cycles
* - MCU cycles for waiting loop for completion of dummy conversion (4)
* - MCU cycles for starting next conversion (2)
* - 5µs delay
* - MCU cycles for enabling pulse (4)
*
* That time is the first half of the puls. So we have to double the time
* for a full pulse. Half pulse for 8MHz MCU clock is about 13.5µs.
*/
/* delay for pulse */
/* MCU cycles for one ADC cycle * 2.5 - MCU cycles for 5µs - 10 */
U_1 = ((MCU_CYCLES_PER_ADC * 25) / 10) - (MCU_CYCLES_PER_US * 5) - 10;
n = (uint8_t)U_1;
/* set up delay timer */
if (SetupDelayTimer(n) == 0) return ESR; /* skip on error */
/*
* charge capacitor with a negative pulse of half length
* pulse: GND -- probe-2 / probe-1 -- Rl -- 5V
*/
ADC_PORT = 0; /* set ADC port to low */
ADMUX = Probe1; /* set input channel to probe-1 & set bandgap ref */
wait10ms(); /* time for voltage stabilization */
ADC_DDR = Probes.Pin_2; /* pull down probe-2 directly */
R_PORT = Probes.Rl_1; /* pull up probe-1 via Rl */
R_DDR = Probes.Rl_1; /* enable resistor */
DelayTimer(); /* wait 1/2 pulse */
R_PORT = 0; /* set resistor port to low */
R_DDR = 0; /* set resistor port to HiZ */
/*
* measurement loop:
* - simulate AC by positive and negative pulses
* - measure start voltage (no load)
* - measure pulse voltage (with load)
*/
n = 255;
while (n > 0)
{
/*
* forward mode, probe-1 only (probe-2 in HiZ mode)
* get voltage at probe-1 (facing Gnd)
* set probes: GND -- probe-1 -- Rl -- 5V / probe-2 -- HiZ
*/
ADC_DDR = Probes.Pin_1; /* pull down probe-1 directly to GND */
R_PORT = Probes.Rl_1; /* pull up probe-1 via Rl */
R_DDR = Probes.Rl_1; /* enable resistor */
ADMUX = Probe1; /* set input channel to probe-1 & set bandgap ref */
wdt_reset(); /* reset watchdog */
/* run dummy conversion for ADMUX change */
ADCSRA = ADC_Mask; /* start conversion */
while (ADCSRA & (1 << ADSC)); /* wait until conversion is done */
/* real conversion */
ADCSRA = ADC_Mask; /* start conversion */
while (ADCSRA & (1 << ADSC)); /* wait until conversion is done */
U_1 = ADCW; /* save ADC value */
/*
* forward mode, positive charging pulse
* get voltage at probe-2 (facing Vcc)
* set probes: GND -- probe-1 / probe-2 -- Rl -- 5V
*/
ADMUX = Probe2; /* set input channel to probe-2 & set bandgap ref */
/* run dummy conversion for ADMUX change */
ADCSRA = ADC_Mask; /* start conversion */
while (ADCSRA & (1 << ADSC)); /* wait until conversion is done */
/* read ADC in the mid of a positive charging pulse */
ADCSRA = ADC_Mask; /* start conversion with next ADC clock cycle */
wait5us();
R_PORT = Probes.Rl_2; /* pull up probe-2 via Rl */
R_DDR = Probes.Rl_2; /* enable resistor */
DelayTimer(); /* wait 1/2 pulse */
DelayTimer(); /* wait another 1/2 pulse */
R_PORT = 0; /* set resistor port to low */
R_DDR = 0; /* set resistor port to HiZ */
while (ADCSRA & (1 << ADSC)); /* wait until conversion is done */
U_2 = ADCW; /* save ADC value */
/*
* prevent runaway of cap's charge
*/
if (U_2 <= 100)
{
/* charge cap a little bit more (positive pulse) */
/* set probes: GND -- probe-1 / probe-2 -- Rl -- 5V */
/* probe-1 is still pulled down directly */
R_PORT = Probes.Rl_2; /* pull up probe-2 via Rl */
R_DDR = Probes.Rl_2; /* enable pull up */
wait2us();
DelayTimer(); /* wait 1/2 pulse */
R_DDR = 0; /* disable any pull up */
R_PORT = 0; /* reset probe resistors */
}
/*
* reverse mode, probe-2 only (probe-1 in HiZ mode)
* get voltage at probe 2 (facing Gnd)
* set probes: GND -- probe-2 -- Rl -- 5V / probe-1 -- HiZ
*/
ADC_DDR = Probes.Pin_2; /* pull down probe-2 directly */
R_PORT = Probes.Rl_2; /* pull up probe-2 via Rl */
R_DDR = Probes.Rl_2; /* enable resistor */
ADMUX = Probe2; /* set input channel to probe-2 & set bandgap ref */
wdt_reset(); /* reset watchdog */
/* run dummy conversion for ADMUX change */
ADCSRA = ADC_Mask; /* start conversion */
while (ADCSRA & (1 << ADSC)); /* wait until conversion is done */
/* real conversion */
ADCSRA = ADC_Mask; /* start conversion */
while (ADCSRA & (1 << ADSC)); /* wait until conversion is done */
U_3 = ADCW; /* save ADC value */
/*
* reverse mode, negative charging pulse
* get voltage at probe-1 (facing Vcc)
* set probes: GND -- probe-2 / probe-1 -- Rl -- 5V
*/
ADMUX = Probe1; /* set input channel to probe-1 & set bandgap ref */
/* run dummy conversion for ADMUX change */
ADCSRA = ADC_Mask; /* start conversion */
while (ADCSRA & (1 << ADSC)); /* wait until conversion is done */
/* read ADC in the mid of a negatve charging pulse */
ADCSRA = ADC_Mask; /* start conversion with next ADC clock cycle */
wait5us();
R_PORT = Probes.Rl_1; /* pull up probe-1 via Rl */
R_DDR = Probes.Rl_1; /* enable resistor */
DelayTimer(); /* wait 1/2 pulse */
DelayTimer(); /* wait another 1/2 pulse */
R_PORT = 0; /* set resistor port to low */
R_DDR = 0; /* set resistor port to HiZ */
while (ADCSRA & (1 << ADSC)); /* wait until conversion is done */
U_4 = ADCW; /* save ADC value */
/*
* prevent runaway of cap's charge
*/
if (U_4 <= 100) /* <= 107mV */
{
/* charge cap a little bit more (negative pulse) */
/* set probes: GND -- probe-2 / probe-1 -- Rl -- 5V */
/* probe-2 is still pulled down directly */
R_PORT = Probes.Rl_1; /* pull up probe-1 via Rl */
R_DDR = Probes.Rl_1; /* enable pull up */
wait2us();
DelayTimer(); /* wait 1/2 pulse */
R_DDR = 0; /* disable any pull up */
R_PORT = 0; /* reset probe resistors */
}
/*
* manage measured values
*/
U_1 += U_3; /* sum of both measurements without pulses/load */
Sum_1 += U_1; /* add to total no-load sum */
U_2 += U_4; /* sum of both measurements with pulses/load */
Sum_2 += U_2; /* add to total with-load sum */
n--; /* next loop run */
}
/*
* process measurements
*/
/* calculate voltage across the DUT */
if (Sum_2 > Sum_1) /* valid measurement */
{
Sum_2 -= Sum_1; /* subtract voltage at DUT's low side (RiL) */
}
else /* invalid measurement */
{
Sum_2 = 0;
}
/*
* calculate ESR
* - ESR = U_ESR / I_ESR
* with U_ESR = (U2 or U4) and I_ESR = (U1 or U3) / RiL
* ESR = (U2 or U4) * RiL / (U1 or U3)
* - since we divide (U2 or U4) by (U1 or U3), we don't need to convert
* the ADC value into a voltage and simply desample the sums.
* - so ESR = Sum_2 * RiL / Sum_1
* - for a resolution of 0.01 Ohms we have to scale RiL to 0.01 Ohms
*/
Value = (uint32_t)(NV.RiL * 10); /* RiL in 0.01 Ohms */
Value *= Sum_2;
Value /= Sum_1;
U_1 = (uint16_t)Value;
/* consider probe resistance */
if (U_1 > NV.RZero)
{
U_1 -= NV.RZero; /* subtract offset */
ESR = U_1; /* we got a valid result */
}
/* update Uref flag for next ADC run */
Cfg.RefFlag = ADC_REF_BANDGAP; /* update flag */
return ESR;
}
#endif
/* ************************************************************************
* capacitance measurements
* ************************************************************************ */
/*
We measure the capacitance by measuring the time needed to charge up the DUT
to a specific voltage using a constant voltage source:
U_c(t) = U_in * (1 - e^(-t/RC))
With ln(e^x) = x we get
C = -t / (R * ln(1 - U_c/U_in)
for an ideal capacitor whithout parallel resistive losses by leakage.
Instead of calculating C directly we'll use pre calculated tables to speed
up things and keep the firmware small. The tables hold the pre-calculated
values of -1/(R * ln(1 - U_c/U_in) for a specific range of U_c, so we just
have to multiply the time with that stored factor to get C.
Large caps:
- R = 680 + 22 (22 is the internal resistance of the MCU for pull-up)
- U_in = 5V
- values are: (-1 / (R * ln(1 - U_c/U_in))) * 10^9n * 10^-2s * 10^-1
- 10^9n for nF scale
- 10^-2s for charge pulses of 10ms each
- 10^-1 internal scale factor (make values fit in uint16_t)
- bc:
- options: -i -l
- define x (u) { return (-1000000 / (702 * l(1 - u/5000))); }
Small caps:
- R = 470k (neglect internal resistance of uC)
- U_in = 5V
- values are: (-1 / (R * ln(1 - U_c/U_in))) * 10^12p * 10^-4
- 10^12p for pF scale
- 10^-4 internal scale factor (make values fit in uint16_t)
- bc:
- options: -i -l
- define x (u) { return (-100000000 / (470000 * l(1 - u/5000))); }
- We could use 10^-3 as internal scale factor to maximize resolution.
*/
/*
* measure cap >4.7µF between two probe pins
*
* requires:
* - Cap: pointer to capacitor data structure
*
* returns:
* - 3 on success
* - 2 if capacitance is too low
* - 1 if capacitance is too high
* - 0 on any problem
*/
uint8_t LargeCap(Capacitor_Type *Cap)
{
uint8_t Flag = 3; /* return value */
uint8_t TempByte; /* temp. value */
uint8_t Mode; /* measurement mode */
int8_t Scale; /* capacitance scale */
uint16_t TempInt; /* temp. value */
uint16_t Pulses; /* number of charging pulses */
uint16_t U_Zero; /* voltage before charging */
uint16_t U_Cap; /* voltage of DUT */
uint16_t U_Drop = 0; /* voltage drop */
uint32_t Raw; /* raw capacitance value */
uint32_t Value; /* corrected capacitance value */
/* set up mode */
Mode = PULL_10MS | PULL_UP; /* start with large caps */
/*
* We charge the DUT with up to 500 pulses each 10ms long until the
* DUT reaches 300mV. The charging is done via Rl. This method is
* suitable for large capacitances from 47uF up to 100mF. If we find a
* lower capacitance we'll switch to 1ms charging pulses and try again
* (4.7µF up to 47µF).
*
* Problem:
* ReadADC() needs about 5ms (44 runs). We charge the DUT for 10ms and
* measure for 5ms. During that time the voltage will drop due to
* resistive losses of the DUT and the measurement itself. So the DUT
* seems to need more time to reach 300mV causing a higher capacitance
* be calculated.
*
* Remark:
* The Analog Input Resistance of the ADC is 100MOhm typically.
*/
large_cap:
/* prepare probes */
DischargeProbes(); /* try to discharge probes */
if (Check.Found == COMP_ERROR) return 0; /* skip on error */
/* set probes: Gnd -- probe-2 / probe-1 -- HiZ */
ADC_PORT = 0; /* set ADC port to low */
ADC_DDR = Probes.Pin_2; /* pull down probe-2 directly */
R_PORT = 0; /* set resistor port to low */
R_DDR = 0; /* set resistor port to HiZ */
U_Zero = ReadU(Probes.ADC_1); /* get zero voltage (noise) */
/* charge DUT with up to 500 pulses until it reaches 300mV */
/* pulse: probe-1 -- Rl -- Vcc */
Pulses = 0;
TempByte = 1;
while (TempByte)
{
Pulses++;
PullProbe(Probes.Rl_1, Mode); /* charging pulse */
U_Cap = ReadU(Probes.ADC_1); /* get voltage */
/* zero offset */
if (U_Cap > U_Zero) /* voltage higher than zero offset */
U_Cap -= U_Zero; /* subtract zero offset */
else /* shouldn't happen but you never know */
U_Cap = 0; /* assume 0V */
/* end loop if charging is too slow */
if ((Pulses == 126) && (U_Cap < 75)) TempByte = 0;
/* end loop if 300mV are reached */
if (U_Cap >= 300) TempByte = 0;
/* end loop if maximum pulses are reached */
if (Pulses == 500) TempByte = 0;
wdt_reset(); /* reset watchdog */
}
/* if 300mV are not reached DUT isn't a cap or much too large (>100mF) */
/* we can ignore that for mid-sized caps */
if (U_Cap < 300)
{
Flag = 1;
}
/* if 1300mV are reached with one pulse, we got a small cap */
if ((Pulses == 1) && (U_Cap > 1300))
{
if (Mode & PULL_10MS) /* 10ms pulses (>47µF) */
{
Mode = PULL_1MS | PULL_UP; /* set mode to 1ms charging pulses (<47µF) */
goto large_cap; /* and re-run */
}
else /* 1ms pulses (<47µF) */
{
Flag = 2; /* signal low capacitance (<4.7µF) */
}
}
/*
* Check if DUT sustains the charge and get the voltage drop.
* - Run for the same time as before (minus the 10ms charging time).
* - This gives us the approximation of the leakage.
* - The charge required for the ADC conversion can be neglected because
* C_S/H is just 14pF (very small vs. DUT).
*/
if (Flag == 3)
{
/* check self-discharging for measuring period */
TempInt = Pulses;
while (TempInt > 0)
{
TempInt--; /* descrease timeout */
U_Drop = ReadU(Probes.ADC_1); /* get voltage */
U_Drop -= U_Zero; /* zero offset */
wdt_reset(); /* reset watchdog */
}
/* calculate voltage drop */
if (U_Cap > U_Drop) U_Drop = U_Cap - U_Drop;
else U_Drop = 0;
/* if voltage drop is too large, consider DUT not to be a cap */
if (U_Drop > 100) Flag = 0;
/*
* Take a second measurement with a specific delay to
* determine the self-discharge leakage current.
*/
U_Zero = ReadU(Probes.ADC_1); /* get start voltage */
if (Mode & PULL_10MS) /* > 47µF */
{
wait1000ms();
}
else /* < 47µF */
{
wait100ms();
}
TempInt = ReadU(Probes.ADC_1); /* get voltage after delay */
/* calculate voltage drop */
if (U_Zero > TempInt) U_Zero -= TempInt;
else U_Zero = 0;
}
/*
* calculate capacitance
* - use factor from pre-calculated LargeCap_table
* - ignore NV.CapZero since it's in the pF range
* - consider voltage drop by ADC and leakage
*/
if (Flag == 3)
{
Scale = -9; /* factor is scaled to nF */
/* get interpolated factor from table */
Raw = GetFactor(U_Cap + U_Drop, TABLE_LARGE_CAP);
Raw *= Pulses; /* C = pulses * factor */
if (Mode & PULL_10MS) Raw *= 10; /* *10 for 10ms charging pulses */