The circuit on a breadboard for test

Some time ago i wrote a theory explaining how the touch sensors work, covering the resistance, the AC Hum and the capacitance touch sensors. Microchip has developed the mTouch(tm) sensing solutions. Part of the mTouch(tm) sensing solutions, is a series of PIC microcontrollers with embedded capacitance module. A list of these PICs can be found in this link.

In this article, i will use the PIC 16F1937, a powerful microcontroller with a 16-channel capacitance module. I will only use the 4 of them, but the idea to expand it to all 16 channels is the same.

Using the capacitance module of the PIC, the circuit is very simple: (click to enlarge)

I will not add a BOM this time. The PIC is the 16F1937. All the job is done by the PIC software. The electrodes are connected to the E1 through E4 inputs. The 4 outputs drive the load. In my case, the load is an LED with a 220 Ohm resistor, but you can interface a transistor to further increase the load.

There are 3 ways to use the capacitance touch module of the PIC. The method used is the frequency change, as described in the theory of capacitance sensors. The 3 different ways mentioned before, have to do with the time base source selection. One way is to use the Timer0 module, the second way is using the Timer2 module, and the third way is using the watchdog timer. All these ways are explained in the Microchip Application Note AN1171, written from Enrique Aleman. The problem with this application note is that it assumes that the reader has plenty of experience using the timer modules and the gate module for timer1. I read it twice and 3 times, and i read the 16F1937 datasheet a couple of times before i can tell that i fully understand the settings. So, i will try to re-explain the method that i use.

For my circuit, i will use the Timer0 module as a time base. Here is how it works: The Timer0 module receives counts from the internal oscillator, through the prescaller. The Tmr0 overflow interrupt is enabled to watch every time that the Tmr0 overflows. This way, we have a fixed time base, because the internal oscillator and the prescaller remain unchanged. The Timer1 module receives counts only as long as the gate module has not detect a tmr0 overflow, and these counts come from the capacitance sensor module. So, when the tmr0 overflows, 2 things happen: First, the tmr1 stops counting, and second the ISR routine is called for further check. This is the rough explanation of this method.

For me, it is more important that you understand how this works, rather than reproducing the circuit. Human creativity has infinite starting points and no limits. So, let's take a closer look. First, the tmr0 module. Here is the portion of the code that i set it up:

	movlw b'11000101'	; I use the capacitance module with TMR0 as timer.
	OPTION			; So i have to set-it up through the Option Register
				; The prescaler is also connected to TMR0.
				; Bit 5: TMR0 Source is internal instruction clock (Fosc/4)
				; Bit 3: Prescaller assigned to TMR0 module
				; Bits <2:0>: Prescaller rate is 1:64

The option register is used to set up Timer0 module. Bits 7 and 6 are irrelevant to the capacitance module. In bit 5, i set Tmr0 to receive counts from the internal oscillator, which is Fosc/4. In my case, the internal oscillator oscillates at 16Mhz, so the pulses driven to the prescaller have a frequency of 4 MHz. Then, the prescaller is assigned to Tmr0 module to further divide these counts by 64. The pulses that finally are delivered to the Tmr0 module, have a frequency of 62.5 KHz. The following drawing is from the PIC16F1937 datasheet, and shows the diagram of Tmr0. With red, i have marked the route of the pulses to the Tmr0 register:

Timer0 is used as a timebase to measure the frequency. The Tmr1 (explained bellow) is used to measure the frequency (go to page 2).

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