It is often desirable to quantify the pulse width of a periodic signal, such as a servo motor, or duty cycle of a pulse-width modulated signal. This code example demonstrates six different methods to measure a pulse and a period, and based on those to calculate the duty cycle, all while using the PIC16F18076.
- PIC16F18076 Data Sheet
- PIC16F18076 Family Product Page
- Various Solutions for Calculating a Pulse and Duty Cycle Application Note
- MPLAB® X IDE 6.0.0 or newer
- MPLAB Xpress IDE (alternative to MPLAB X IDE)
- MPLAB XC8 Compiler 2.36 or a newer compiler
- Microchip PIC16F1xxxx Series Device Support pack 1.15.191 or newer Device Pack
- PIC16F18076 Curiosity Nano (EV53Z50A) (x2)
Connect two PIC16F18076 Curiousity Nano boards together following the diagram in Figure 1.
Figure 1 - Building the Circuit
Make sure to connect the Universal Asynchronous Receiver Transmitter (UART) pin to a logic analyzer so that the results can be viewed.
-
Download, install and open MPLAB X IDE version 6.0.0 (or newer)
-
Download and install the XC8 C-Compiler version 2.36 (or newer)
-
Download and open the
pic16f18076-calculating-pulse-and-duty-cycle-mplab-melody.Xproject, as shown in Figure 2
Figure 2 - Open Project Menu in MPLAB X IDE
Once the project is opened, select either the 'free' or 'pro' configuration as shown in Figure 3.
Figure 3 - Select Project Configuration
Program the device by clicking Make and Program Device Main Project from the taskbar at the top (Figure 4).
Figure 4 - Make and Program Device Main Project Button
Wait for the Output tab to show 'Programming complete' (Figure 5), then proceed to the Operation section below.
Figure 5 - Programming Complete
This example uses a button (SW0) to toggle between various methods of measuring the pulse width and duty cycle (Figure 6).
Figure 6 - PIC16F18076 Curiousity Nano Board SW0 Button
Pressing the button changes the method of measurement following the order in the list below (after #9 is starts over at #1):
- Pulse Generator
- Timer1 Gate
- Configurable Logic Cell (CLC) Numerically Controlled Oscillator (NCO) 1
- CLC NCO 2
- CLC NCO 3
- Capture/Compare/PWM (CCP)
- Interrupt-On-Change (IOC) with Timer
- IOC without Timer
- Polled Input
The Pulse Generator mode is the default mode for this example. It generates a 1 MHz signal to the RA3 pin with varying duty cycles, ranging from 10% to 90% at 10% intervals on a constant loop. This signal will output the pulse to pin RA3. A waveform example for the pulse generator can be seen in Figure 7.
Figure 7 - Pulse Generator Waveform Example
The Timer1 (TMR1) Gate is the first measuring method, using the Timer1 module to measure the pulse width and duty cycle. The pulse (RB5 pin) is measured by having the Timer1 Gate close, thus connecting the Timer1 clock source to the Timer1 counter when the pulse it high. The period is measured similarly, with different interrupt flags. The duty cycle is then calculated by taking the pulse and dividing it by the period. The code outputs an 8-bit value (0-255) which correlates to 0%-100% (i.e., 0x80 or 127 equals 50%). Refer to the Timer1_Gate.c file for more details. A waveform example for the Timer1 Gate can be seen in Figure 8.
Figure 8 - Timer1 Gate Waveform Example
The CLC NCO modules are used in the second, third, and fourth measuring methods for this example. Each of these methods use either two or three CLC modules, the NCO module and either the Timer1 module and/or the Timer0 (TMR0) module.
The CLC NCO method uses the CLC1, CLC2, NCO1 and Timer1 modules. The pulse (RB5 pin) is measured by the NCO1 module which is clocked by a 4-input AND gate that is configured to CLC1. The period is measured by starting and stopping Timer1 based on various interrupt flags. The duty cycle is then calculated by taking the pulse and dividing it by the period. The code outputs an 8-bit value (0-255) which correlates to 0%-100% (i.e., 0x80 or 127 equals 50%). Refer to the CLC_NCO.c file for more details. A waveform example for the CLC NCO 1 can be seen in Figure 9.
Figure 9 - CLC NCO 1 Waveform Example
This method has an additional output pin (other than UART) that provides information on the CLC1OUT status. This signal can be monitored, in addition to the other signals, by the logic analyzer.
The CLC NCO method uses the CLC1, CLC2, CLC3, NCO1 and Timer1 modules. The pulse (RB5 pin) is measured by the NCO1 module which is clocked by a 4-input AND gate that is configured to CLC1. The period is measured by starting and stopping Timer1 based on various interrupt flags. The duty cycle is then calculated by taking the pulse and dividing it by the period. The code outputs an 8-bit value (0-255) which correlates to 0%-100% (i.e., 0x80 or 127 equals 50%). Refer to the CLC_NCO.c file for more details. A waveform example for the CLC NCO 2 can be seen in Figure 10.
Figure 10 - CLC NCO 2 Waveform Example
This method has additional output pins (other than UART) that provide information on the CLC1OUT status and the CLC2OUT status. These signals can be monitored, in addition to the other signals, by the logic analyzer.
The CLC NCO method uses the CLC1, CLC2, CLC3, NCO1, Timer0 and Timer1 modules. The pulse (RB5 pin) is measured by the Timer0 module which is clocked by a 4-input AND date that is configured to CLC1. The period is measured by starting and stopping Timer1 based on various interrupt flags. The duty cycle is then calculated by taking the pulse and dividing it by the period. The code outputs an 8-bit value (0-255) which correlates to 0%-100% (i.e., 0x80 or 127 equals 50%). Refer to the CLC_NCO.c file for more details. A waveform example for the CLC NCO 3 can be seen in Figure 11.
Figure 11 - CLC NCO 3 Waveform Example
This method has additional output pins (other than UART) that provide information on the status of CLC1OUT, CLC2OUT and CLC3OUT. These signals can be monitored, in addition to the other signals, by the logic analyzer.
The CCP is the fifth measuring method, using the CCP1, CCP2 and Timer1 modules to measure the pulse width and duty cycle. The pulse (RB5 pin) is measured by using the CCP2 interrupt flag to save two different values (derived from the Timer1 module) and then to take the difference between them. The period is measured in the same way, using the CCP1 interrupt flag. The duty cycle is then calculated by taking the pulse and dividing it by the period. The code outputs an 8-bit value (0-255) which correlates to 0%-100% (i.e., 0x80 or 127 equals 50%). Refer to the CCP.c file for more details. A waveform example for the CCP method can be seen in Figure 12.
Figure 12 - CCP Waveform Example
The IOC with Timer is the sixth measuring method using the IOC and Timer1 modules to measure the pulse width and duty cycle. The pulse and period (RB5 pin) are measured by reading the Timer1 value at various times that are determined by the IOC bit values. The duty cycle is then calculated by taking the pulse and dividing it by the period. The code outputs an 8-bit value (0-255) which correlates to 0%-100% (i.e., 0x80 or 127 equals 50%). Refer to the IOC_w_Timer.c file for more details. A waveform example for the IOC with Timer can be seen in Figure 13.
Figure 13 - IOC with Timer Waveform Example
The IOC without Timer is the seventh measuring method using the IOC module to measure the pulse width and duty cycle. The pulse and period (RB5 pin) are measured by incrementing the pulse or period value depending on the state of the IOC bit values. The duty cycle is then calculated by taking the pulse and dividing it by the period. The code outputs an 8-bit value (0-255) which correlates to 0%-100% (i.e., 0x80 or 127 equals 50%). Refer to the IOC_wo_Timer.c file for more details. A waveform example for the IOC without Timer can be seen in Figure 14.
Figure 14 - IOC without Timer Waveform Example
The Polled Input is the eighth and final measuring method, using only pin settings instead of modules to measure the pulse width and duty cycle. The pulse and period (RB5 pin) are measured by incrementing the pulse or period value depending on the state of the input signal pin. The duty cycle is then calculated by taking the pulse and dividing it by the period. The code outputs an 8-bit value (0-255) which correlates to 0%-100% (i.e., 0x80 or 127 equals 50%). Refer to the Polled_Input.c file for more details. A waveform example for the Polled Input can be seen in Figure 15.
Figure 15 - Polled Input Waveform Example
The image below shows what can be seen on a logic analyzer when all modes are cycled through once.
Figure 16 - All Button Modes
Refer to AN3324 - Various Solutions for Calculating a Pulse and Duty Cycle for more in-depth information on each measurement method.
This example demonstrated several methods of how to measure the pulse width and duty cycle of a pulse on the PIC16F18076 Curiosity Nano board.