In Part 1, we briefly covered how to optimize power consumption in capacitive sensing system, utilizing sensor ganging with a capacitive sensing controller, and use cases of sensor ganging. In part II, we will cover trade-offs between response time and power consumption, as well as other issues resolved by a ganged sensor.
Response Time vs. Power Consumption
Before discussing the advantages of sensor ganging, let us discuss the tradeoffs between response time and power consumption in a capacitive sensing system. The response time of a system is defined as the time required by a system to give a valid output from the time the corresponding input is received.
In capacitive sensing applications, it is not easy to optimize both power consumption response time as it takes a considerable amount of time to scan sensors. The refresh interval can be varied to optimize either power consumption or response time. But it is not always possible to get the desired level of optimization by controlling the refresh interval alone. If the device is optimized for power consumption, then response time will be slower, and the opposite is also true.
Another technique is used to optimize response time and power consumption. When a user interacts with the device, the capacitive sensing system can be optimized to respond quickly and when the user does not touch the buttons, the device can be optimized for power consumption.
To achieve the optimization of these two key parameters – power consumption and response time – the refresh interval needs to be adjusted. For optimizing response time, the sensors need to be scanned at a high rate and hence the refresh interval needs to be small. Let us call this quick scan mode. For optimizing power consumption, the sensors need to be scanned at a lower rate as the device needs to sleep for more time, hence the refresh interval needs to be larger. Let us call this slow scan mode .
Power consumption varies between these two modes. The scan modes can be switched between the two modes dynamically to optimize power consumption and response time. When the user does not touch buttons, (i.e., when the UI is idle) the capacitive sensing controller is put into slow scan mode thus optimizing the power. When the user has touched the buttons the mode is changed to quick scan mode to optimize response time. In this section of this article, we will see how much power can be optimized with ganged sensors when combined with this technique of switching between quick scan mode and slow scan mode.
Problems solved by a ganged sensor
Now let us take an example application and see how sensor ganging helps in power optimization. We will also use dynamic scan mode switching as discussed in the previous section to optimize both response time and power consumption. Here is the procedure for average power consumption calculation:
Power consumption with dynamic scan mode switching Consider a capacitive sensing user interface in a digital photo frame with six buttons, for example. Let us take quick scan mode refresh interval as 50ms and slow scan mode refresh interval as 125ms. Let us assume that the total sensor scan time for six buttons is 9ms.
In the above example, with a ganged sensor, power consumption is reduced by 32%.