The Timer1 is a 16-bit counter, and is used to measure the frequency. The Tmr1H and Tmr1L registers (each one 8 bits, both make the Tmr1H:Tmr1L 16 bit register), are increased by the capacitance sensor module. This means, that the rate of increment depends on the frequency of the capacitance module, which eventually depends on the capacitance of the electrode. Larger capacitance (if for example the electrode is touched), means lower frequency which means slower rate of change for the Tmr1 counter. Here is the setup code:

	movlw b'11000100'	; Setup TMR1 module Control register
	movwf T1CON		; Bits <7:6>: Timer 1 source is Capacitive Sensing Oscillator
				; Bits <5:4>: Prescaller is 1:1
				; Bit 3: LP oscillatos is deactivated
				; Bit 2: No synchronization
				; Bit 0: Timer 1 is stopped

The T1Con register is responsible for the operation of Timer1. In bits 7 and 6 i set the capacitance module as the pulse source for the counter. Then, i set the prescaller to 1:1, which means that no further division will be performed. The LP oscillator is not needed so i turn it off, and also i do not want synchronization. Then, i keep the Timer 1 module stopped for a while. I turn it on after a few more lines of code. The following drawing is from the datasheet of the PIC16F1937. The red route shows how the pulses are delivered from the capacitance module to the Tmr1 registers. The portion on top of this drawing is the gate for the tmr1, so i have fade-it out. I will explain this later.

Everything is pretty straight forward. Let's go to something little bit more stiffer.

Look at the drawing above, which illustrates the Tmr1 module. Notice that before the Tmr1H:Tmr1L registers, there is a gating module. The output Q of it, is connected to the Tmr1 registers, at the D input the pulses from the capacitance module arrive, and there is also another input, the "EN" from the word "Enabled". This is the gate of the Tmr1. As long as the "Enabled" input is kept high, the pulses normally arrive to the 16-bit counter. But if the "Enabled" is low, no matter what, the pulses will NOT increase the 16-bit registers. This is one feature that i will use to measure the frequency. And here is the portion of the code in which i set the gate module:

	movlw b'11100001'	; Setup Timer 1 Gate module
	movwf T1GCON		; Bit 7: Gate controls the counting function of TMR1
				; Bit 6: Gate polarity is Active HIGH
				; Bit 5: Gate input is through Toggle FF
				; Bits <4:3>: No single pulse mode
				; Bits <1:0>: Gate source is Timer0 overflow

The T1Gcon is the responsible register for the Tmr1 Gate control. First of all, i enable the gate by setting bit 7, otherwise the gate would have no affect. Then i set the polarity of the gate to active-high. This means that the Gate will be activated on a Low to High pulse, which is exactly what we want, as the Tmr0 overflow sends such a pulse upon overflow. Then i enable the toggle flip flop and disable the Single pulse mode. What this means is that, the gate output will toggle upon each Timer0 overflow. If you do not clearly understand this, don't worry. I explain it later. Finally, i set the gate input to arrive from the Tmr0 overflow, which is the time base. Here is the route of the Gate:

Now, let's see an example of the above setup. Suppose that the Tmr1H:Tmr1L registers are cleared, as well as the Tmr0 register is cleared. And also, suppose that the toggle FF output of the gate is HIGH, which means that the pulses will indeed arrive to the Tmr1 count. From this point, the operation begins. The pulses from the capacitance module are delivered to the Tmr1 16 bit counter, which is increased accordingly. Simultaneously, the pulses from the internal oscillator are delivered to Tmr0 register through he prescaller. Then, after a while (very short while), the Tmr0 register overflows from 0xFF to 0x00. When this occurs, a pulse will be delivered from the Tmr0 module to the Tmr1 Gate input. This pulse will toggle the flip flop, and its output will become LOW. Eventually, the "Enabled" input of the Gate module right before the Tmr1 16 bit register will also become LOW, and no further pulses will be delivered to the Tmr1 registers. This means that the count will stop!

But the Tmr0 overflow will also cause an interrupt, the Tmr0 interrupt. So, the program counter will go to 0x04 origin, where the Interrupt Service Routine is located. During this routine, the Tmr1 16-bit value is acquired and compared with a Press and Release threshold level, to determine if the electrode is pressed or released. I will not go into details for this procedure, first because the code is well documented to understand it, and second because you can read another article that i wrote, in which i explain how this works. You can find this article here - PIC Frequency-Change Capacitance Touch Button.

Now lets see what happens when the ISR ends. First, we need to clear the Tmr0 and Tmr1 registers to be ready for the next cycle. But the toggle flip flop of the Tmr1 gate is now LOW, which means that no pulses will arrive to the Tmr1. As there is no immediate way to set the FF again, i do this trick. I put the value 0xFF to Tmr0, and wait until it overflows. The Tmr0IF is NOT cleared, so, when the overflow occurs, the ISR is NOT called again, but a pulse still arrives from the Tmr0 module to the flip flop, toggling its state back to HIGH. Then, i clear the Tmr0IF to re-enable the Tmr0 interrupt, and the PIC is ready to measure the frequency once again.

I hope that you clearly understand how it works. Here is the firmware for further study:

PIC Touch Button Capacitance module and Tmr0 timebase - Assembly listing - V1.0

And the Hex file:

PIC Touch Button Capacitance module and Tmr0 timebase - Hex file - V1.0

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