In many electronics products such as consumer, home appliance, automotive, industrial etc., capacitive touch buttons are fast replacing traditional mechanical buttons. Although capacitive buttons have many advantages over mechanical buttons, there are certain parameters that need to be considered by a system design engineer while building a capacitive sensing system. These parameters are:
- Signal-to-noise ration (SNR)
- Response time
- Power consumption
SNR is critical to ensure the robust performance of a capacitive sensing system as capacitive sensors are vulnerable to noise that is internal and external to the controller. The focus of this article is on the remaining two parameters.
Response time indicates how fast a capacitive sensor responds to a touch. There is often a tradeoff between power consumption and response time. In this article, we will discuss response time considerations that a designer has to make while trying to optimize power consumption.
Capacitive sensors need to be scanned for specific time (called scan time), based on such sensor characteristics as parasitic capacitance and sensitivity to touch. Scan time is a major contributor to the power consumption of a capacitive sensing controller. Power consumption optimization is especially critical for battery-operated devices like mobile phones and wearable devices including heart rate monitors. There are various methods to optimize power consumption, which include optimizing scan time as well as the rate at which sensors are scanned. In this article we will introduce and explain one of the salient methods called sensor ganging for optimizing the power consumption of a capacitive sensing system.
Optimizing power consumption
The important factors deciding power consumption are the scan time of sensors and the rate at which the sensors are scanned. Sleep current values are typically much less compared to active current values. Thus, when a capacitive sensing system is not being used, the capacitive sensing controller can be put into sleep mode so that the average current consumption is reduced. In order to optimize power in capacitive sensing system, a common technique of scan-sleep-scan-sleep is used (See Figure 1). With this technique, all the sensors are scanned and then the controller is put into a low power sleep mode. This is one cycle and this cycle is repeated. One cycle of scan-sleep is called one ‘refresh interval’. Each refresh interval is comprised of active time and sleep time. The ‘active time’ includes scan time, sensor data processing, and post sensor scan activities such as controlling feedback mechanisms like LEDs and a buzzer. The scan time of sensors forms a major portion of the active time.
Power consumption can be reduced by:
- Shortening the active time i.e., by reducing the scan time or processing time post sensor scan
- Reducing the active current for a given active time
- Increasing the sleep time
Sensor ganging is a techniques that reduces the active time of a capacitive sensing controller and hence reduces the power consumption of the controller. As the number of capacitive sensors increases, for a constant refresh interval, power consumption increases for the same number of sensors, if the refresh interval is decreased then power consumption increases. For a given number of sensors, in order to achieve low power, it may be necessary to increase the refresh interval. However, this action may affect the response time of the sensors. To achieve a good balance between response time and power consumption, you can combine all the sensors and scan them as a single sensor. This is called sensor ganging. The gang of sensors is considered as a single sensor and the capacitive sensing algorithm scans the ganged sensor as one sensor when individual sensors are ganged. Once a touch is detected and confirmed, the sensors are disconnected and scanned individually.
Implementing sensor ganging is possible in devices such as Cypress’ PSoC in which the individual sensors can be connected to a global analog mux bus. In a mixed-signal device like PSoC 4, an internal analog mux bus can be used to connect more than one sensors to the internal CapSense block just in the firmware. A reference Design Guide covering the analog mux bus, and how capacitive sensors are connected to analog mux bus is available at the end of this article.