LDC211x and LDC3114 Internal Algorithm Functionality
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1 Introduction
The Texas Instruments LDC211x and LDC3114 devices can monitor changes in a sensor resonant frequency to determine whether a stimuli corresponds to a user interaction such as a button press. Refer to the Inductive Touch System Design Guide for HMI Button Applications application report for details on how to construct a mechanical system suitable for inductive sensing button applications, sensor design guidance, and an overview of device configuration. The LDC211x and LDC3114 use several internal algorithms to compensate for environmental shifts, determine whether the sensor signal change corresponds to a button press, and also to correct for mechanical cross-talk between multiple buttons. This document details the operation and configuration of the algorithms.
2 Scan Rate and Sampling Interval
Once the LDC211x or LDC3114 powers on, it continuously samples the enabled channels and returns to as shown in Figure 2-1. The active channels, sample time interval, and button scan rate are all configurable parameters. Refer to Table 2-1 for a listing of the device parameters that control the Scan Rate and Sample Interval. A higher Scan Rate produces a more responsive interface, but generally at the cost of higher average supply current consumption. Note that the scan rate can vary by up to ±30% from the nominal value across devices.
Figure 2-1 Sample Interval and Scan
Rate| Device Parameter | Configuration Effect | Details |
|---|---|---|
| SENCYCx | Button Sample Interval for channel x | Each channel has dedicated setting. Sensor frequency is used to determine proper setting. |
| CNTSCx | Button Sample Interval for channel x | Each channel has dedicated setting. Sensor frequency, LCDIV, and SENCYCx is used to determine proper setting |
| LCDIV | Button Sample interval for all channels | Common setting for all channels |
| LPWRB pin | Button Scan Interval | When set High, device samples based on NPSR value. When set Low, device sample based on LPSR value. |
| NPSR | Button Scan Interval | When LPWRB pin is High, this field sets the device scan rate from 10 sps to 80 sps. |
| LPSR | Button Scan Interval | When LPWRB pin is Low, this field sets the device scan rate from 0.625 sps to 5 sps. |
The Button Sample Interval is a function of the SENCYCx, LCDIV, and the Sensor Oscillation Frequency. The sample time, tSAMPLE is set by:
tSAMPLE = 128 × (SENCYCx+1)×2LCDIV ÷ ƒSENSORx
The recommended sample interval is 1 ms, as this provides a good balance on noise vs. supply current. Shorter sample intervals may have reduced SNR. Variations in sensor frequency greater than 2.5% may need to use different settings for SENCYCx. If the sensor frequency increases significantly and the SENCYCx & LCDIV settings are not changed, then a reduction in sensitivity can occur. Alternatively, if the sensor frequency decreases by ~30% from the configured frequency, it is possible for internal counters in the device to over-range, resulting in improper operation.
2.1 Low Power Mode and Normal Power Mode
The LDC211x and the LDC3114 operate in one of two modes – Low Power Mode and Normal Power Mode. In Low Power Mode, the maximum supported scan rate is from 0.625 to 5 sps. In Normal Power Mode, the scan rate is 10 to 80 sps. The primary difference between the two modes is the Baseline Increment, which is discussed below. The buttons enabled in Low Power Mode must be a subset of the buttons enabled in Normal Power Mode.