Product Info

LM746 Bluetooth® Dual Mode Audio Module
Standalone (With Embedded Bluetooth® 5 Compliant Stack)
The overall system-level specification for the capacitive touch sensor interface on the LM746 Module is:
6 inputs multiplexed in to 1 touch sensor on the front-end
Capacitances of 0pF to 50pF measured with a resolution of 4fF, where a touch is assumed to be between ±50fF and ±1pF
Each reading takes 172μs
6 pads read every 1.03ms
System auto-calibrates to remove parasitic and environmental eects including
PCB construction
Temperature
Humidity
Works in normal and deep sleep modes
System current is approximately 50μA from the battery
The touch sensor also functions like a PIO
The system block diagram in Figure 6.1 highlights the top-level architecture for the capacitive touch sensor interface, it consists of:
Capacitive range control
Sets the rough capacitance of the touch sensor pad, which is product dependent
Splits into 4 integrated capacitors
The VM selects which capacitors are enabled, i.e. the range capacitance
Sampling front end
An internal capacitance is trimmed by the digital state machine ensuring
Touch Capacitance = Range Capacitance + Internal Capacitance
When the internal capacitance is correctly trimmed: The sense voltage is 0V
A touch changes the touch capacitance, which then changes the sense voltage
ADC: Uses a successive approximation, charge redistribution ADC Clocked at 64kHz
9-bit resolution, where LSB is ±2fF and full range is ±1pF
The internal capacitance is a 7-bit variable capacitor with 114fF steps and 14.5pF range
Figure 6.1: Capacitive Touch Sensor Block Diagram
Figure 6.1 shows the system block diagram for the capacitive touch sensor interface. The interface depends on the capacitive touch sensor type.
Therefore the overall control of the capacitive touch sensor interface resides in the VM, so it is easily modified in each end-user application.
Capacitive
Range Control
Sampling
Font-end
Digital
Processing
Firmware
CAP_SENSE[0]
CAP_SENSE[1]
CAP_SENSE[2]
CAP_SENSE[3]
CAP_SENSE[4]
CAP_SENSE[5]
VM
ADC
Part No
Product
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LM746
LM746
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