Version: **
Translation - Japanese: ファームウェアによるCapSenseセンサーの相互容量計算 - KBA222425 - Community Translated (JA)
Question:
How do you calculate the mutual capacitance of a CapSense sensor, through firmware in CapSense Component version 3.0 or above?
Answer:
Raw counts are a direct measure of the mutual capacitance (Cm). The raw counts obtained can be used to calculate the mutual capacitance of sensor, by following these steps:
1. Start the CapSense Component; scan and process all the CapSense widgets
2. Calculate the maximum counts using the frequency of the modulator clock, number of sub conversions, and frequency of the transmitter clock.
Max_count =
( Modulator_clock_frequency * Number_of_sub_conversions )
Transmitter_clock_frequency
3. Calculate the hardware raw counts using the max counts (obtained in step 1) and raw counts processed by the CapSense Component.
Raw_count_hardware = Max_count – Raw_count
4. Calculate the mutual capacitance using the max counts (from step 1), hardware raw counts (from step 2), IDAC current, frequency, and amplitude of the transmitter clock signal.
Raw_count_hardware =
( 2 * Max_count * Mutual_capacitance * Transmitter_voltage * Transmitter_clock_frequency )
IDAC_current
include "project.h"
#define TRANSMITTER_VOLTAGE (5u) /* unit - voltage */
#define IDAC_GAIN (300u) /* unit – nano Amperes/bit */
int main(void)
{
CyGlobalIntEnable; /* Enable global interrupts. */
uint32 raw_count, modulator_clock_divider, sense_clock_divider, sub_conversions, IDAC_code, hardware_raw_count, max_count, mutual_capacitance;
/* Start the CapSense component */
CapSense_Start();
/* Scan all the CapSense widgets */
CapSense_ScanAllWidgets();
for(;;)
{
/* Check if CapSense scan is completed*/
if (!CapSense_IsBusy())
{
/* Process all the CapSense widgets */
CapSense_ProcessAllWidgets();
/* Calculation of max counts for button 0*/
/* API to get the value of modulator clock divider = HF_CLK_FREQ / MOD_CLK_FREQ */
CapSense_GetParam (CapSense_MOD_CSX_CLK_PARAM_ID , &modulator_clock_divider);
/* API to get the value of sense clock divider = MOD_CLK_FREQ / SNS_CLK_FREQ */
CapSense_GetParam (CapSense_BUTTON0_SNS_CLK_PARAM_ID, &sense_clock_divider);
/* API to get the number of sub conversions */
CapSense_GetParam (CapSense_BUTTON0_RESOLUTION_PARAM_ID , &sub_conversions);
max_count = sense_clock_divider * sub_conversions ;
/* Calculation of hardware raw counts for button 0 */
/* API to get the value of raw counts */
CapSense_GetParam (CapSense_BUTTON0_RX0_RAW0_PARAM_ID , &raw_count);
hardware_raw_count = max_count-raw_count ;
/* Calculation of mutual capacitance for button 0 */
/* API to get the IDAC code */
CapSense_GetParam (CapSense_BUTTON0_RX0_IDAC_COMP0_PARAM_ID , &IDAC_code);
mutual_capacitance = (hardware_raw_count * IDAC_code * IDAC_GAIN * sense_clock_divider * modulator_clock_divider) / (2 * max_count * TRANSMITTER_VOLTAGE * CYDEV_BCLK__HFCLK__MHZ);
/* Scan all the CapSense widgets*/
CapSense_ScanAllWidgets();
}
}
}
The mutual capacitance is expected to be in the range of thousands of femto farads under the following conditions:
HF_clock_fequency = 48 MHz
Modulator_clock_frequency = 48 MHz
Sense_clock_frequency = 300 kHz
Transmitter_voltage = 5 V
IDAC_gain = 300 nA/ bit
IDAC code= 32
Number_of_sub_conversions = 100
Modulator_clock_divider = 1
Sense clock divider = 160
As an example, if the raw counts obtained are 6200, then
Raw_count_hardware = 16000 – 6200= 9800
Mutual_capacitance =
(9800*300*20*160)
( 2*16000*5*48 )
= 1225 fF
