The Timer2/4/6 modules are 8-bit timers that incorporate the following features:
- 8-bit Timer and Period registers (TMR2 and PR2)
- Readable and writable (both registers)
- Software programmable prescaler (1:1, 1:4, 1:16, and 1:64)
- Software programmable postscaler (1:1 to 1:16)
- Interrupt on TMR2 match with PR2
- Optional use as the shift clock for the MSSP module
Some PIC® MCU Devices only have a Timer2. Timers 4 and 6 are just duplicates of the the Timer2 peripheral. By adding more of this type of timer, a device can control multiple PWM outputs from different clock bases.
Below is a simplified block diagram of the Timer2.
Timer2 has many features built in but its main feature is the ability to compare its contents to a fixed values stored in a separate register (PR2). When the two values match, then different events can be triggered such as toggling a PWM pin from a high to low state.
Timer2 is driven by the internal instruction clock (FOSC/4). The Timer2 register (TMR2) increments on each clock edge. A prescaler on the clock input allows direct input (1:1), divide-by-4, divide-by-16 and on some devices divide-by-64 prescale options. These options are selected by the prescaler control bits, T2CKPS of the Timer2 Control register (T2CON). The value of TMR2 is compared to that of the Period register (PR2) on each clock cycle. When the two values match, the comparator generates a match signal as the timer output for other peripherals to use as a time base. A Timer2 interrupt can also be triggered by the match. The match signal becomes the input to an optional postscaler and also resets the value of TMR2 to 00h on the next cycle.
The TMR2 and PR2 registers are both directly readable and writable. The TMR2 register is cleared on any device Reset, whereas the PR2 register initializes to FFh. Both the prescaler and postscaler counters are cleared on the following events:
- a write to the TMR2 register
- a write to the T2CON register
- Power-on Reset (POR)
- Brown-out Reset (BOR)
- MCLR Reset
- Watchdog Timer (WDT) Reset
- Stack Overflow Reset
- Stack Underflow Reset
- RESET Instruction
Timer2 can be turned on and off via the TMR2ON bit of the Timer2 Control Register.
Timer2 can generate an interrupt off the TMR2 and PR2 register match. That match signal can also feed a postscaler to delay the number of matches required to initiate a Timer2 interrupt. The output of the postscaler sets the Timer2 Interrupt Flag bit (TMR2IF) of the Peripheral Interrupt Register (PIR1). The interrupt is enabled by setting the TMR2 Match Interrupt Enable bit (TMR2IE) of the Peripheral Interrupt Enable (PIE1) register.
0000 = 1:1
0001 = 1:2
0010 = 1:3
1111 = 1:16
The postscaler has a range of 1:1 through 1:16 and is selected by the Timer Output Postscaler Select bits (T2OUTPS) of the Timer2 Control Register (T2CON).
Timer2/4/6 Output Options
The output of the TMR2 match signal is available directly to the CCP modules, where it is typically used as a time base for operations in PWM mode. Having multiple Timer2 style time bases (i.e. Timer4, Timer6) can offer multiple unique PWM signals from a single PIC MCU Device.
The TMR2 match signal can also be directed to the Master Synchronous Serial Port (MSSP) to act as the shift clock source when the MSSP is operating in SPI mode.
Timer2/4/6 Sleep Mode
The Timer2 timers cannot be operated while the processor is in Sleep mode. The contents of the TMR2 and PR2 registers will remain unchanged while the processor is in Sleep mode.
Timer2/4/6 Example using MPLAB Code Configurator
The MPLAB Code Configurator (MCC) makes setting up a 10-bit PWM peripheral easy. The steps include setting up the I/O, Timer 2 and PWM module to make it run. The MCC will automatically generate the code to load the proper registers and initialize the proper values to produce the desired PWM signal.
The best way to show how this is done is through a simple example. A PIC16F1825 Capture/Comparae/PWM peripheral will be configured to create a PWM signal at: 500Hz, 50% duty cycle using a 4Mhz system clock and 1:16 prescaler.
MCC CCP:PWM Setup
The first step after launching the MCC within MPLAB X® is to select the peripherals we will use and setup the PWM.
The three resources required are the System, TMR2::Timer and the CCP3:PWM modules with the MCC list of options.
The System is where the oscillator speed is selected and any changes to the configuration settings you may need. The 4 MHz internal oscillator is selected as shown in the picture below.
Timer 2 Setup
Timer 2 uses the oscillator selected in the System section to adjust the Timer 2 period. The time of 2.0 milliseconds is entered for the period to yield a 500 Hz frequency. The prescaler is selected as 1:16 from the drop-down menu. The Start Timer After Initialization box is also checked. This will start the timer running and also the PWM signal after the PIC16F1825 finishes initializing all the peripherals.
By selecting the CCP3:PWM the MCC automatically selects the I/O pin RA2 in the I/O selection window. The RA2 pin actually shows up with the label CCP3 in green to show that the CCP3 peripheral now controls the I/O pin.
The CCP3:PWM setup screen is where the Duty Cycle is selected and 50 is entered for 50%.
PWM period and frequency are displayed in this window as well based on the Timer 2 selection window.
Generate CodeWhen all the setup screens are complete the MCC Generate Code button is clicked and the MCC produces the software files for the project. The MCC will produce a MAIN.C file that contains a System Initialize function as it's only component.
The System_Initialize function is placed in a file named MCC.C.
System_Initialize calls four functions:
The OSCILLATOR_Initialize function takes the Oscillator Settings selected and sets up the proper registers for the 4 MHz internal oscillator.
The PIN_MANAGER_Initialize function sets the registers for the I/O pins.
The TMR2_Initialize function sets the registers for the Timer 2 settings selected including the prescaler and PR2 value.
The PWM3_Initialize function selects the settings for the 50% duty cycle value. Notice the CCP3RL register is loaded with the proper value to create the proper high time of the 50% duty cycle.
The code is then compiled within MPLABX IDE environment and programmed into the PIC16F1825. The device will start operating as soon as it's powered up. Timer 2 will start running immediately after the initialization phase of the code. The results are shown on the oscilloscope screen capture below. The screen capture shows, in the measurement section, a period of 2 milliseconds and frequency of 500 Hz as we expected. Each pulse is an equal 1 milliseconds off the center of the signal for a perfect 50% duty cycle.