NU32v2: Counters and Timers
Timers are the simplest way to generate accurate time-based events such as flashing an LED at a given frequency. They are also useful for counting external pulses or accurate timing measurements of events.
The PIC32 is equipped with 5 16-bit timers (Timer1, Timer2, Timer3, Timer4, and Timer5). The basic idea of a timer is that it increments based on a clock signal.
In software, the user selects whether the timer increments using an internal clock or an external clock. The internal clock is based on the frequency of the peripheral bus. For the NU32v2, the peripheral bus frequency is set at 80 MHz. The external clock would be used if you wanted to count external pulses such as from an encoder. Each of the timers has a pin labeled 'TXCK' that can be used for the external clock. Additionally, the user selects a prescalar that determines how many pulses the timer receives from the clock before it adds one to the existing value.
Each timer can be configured to generate interrupts at a given priority.
The PIC32 has two types of timers:
- Timer A: (Timer1) is an asynchronous timer with a built in oscillator, can operate in sleep mode and has prescalars of 1:1, 1:8, 1:64, and 1:256
- Timer B: (Timer2, Timer3, Timer4, Timer5) has the ability to form a a 32-bit timer, has prescalars of 1:1, 1:2, 1:4, 1:16, 1:32, 1:64 and 1:256 and can have event triggers.
The functions of the DIO pins are controlled by special function registers (SFRs). Each of these SFRs has 32 bits on the PIC32, but for many, only 16 of the bits are relevant. Depending on whether these bits are set to 0 or 1, different actions or settings are performed for Timer x:
- TxCON: This SFR sets up the timer. There are many different ways to configure the timers. The main options for TxCON are to turn on or off the timer, choose the pre-scalar, specify if the clock is internal or external and change mode to 32-bit timer. Other options can be seen on the PIC32 reference manual.
- TMRx: This register stores the 16-bit counts. It resets to 0 when it reaches the number stored in PRx.
- PRx: 16-bit value to reset TMRx.
Timer interrupts are controlled with the following SFRs:
- TxIE: This SFR sets up the interrupt associated with the timer.
- TxIF: This SFR corresponds to the interrupt Flag Status bit which is either 0 or 1 depending on whether or not the interrupt has been generated.
- TxIP: This SFR sets up the priority of the interrupt associated with the timer.
- TxIS: This SFR sets up the subpriority of the interrupt.
Given a clock source (internal or external) and a prescalar (1:n), the timer increments by 1 every n pulses from the clock. The timer is reset to zero when the timer value equals the PRx value. If interrupts are enabled, the timer interrupt is generated when the timer is reset. This means that PRx determines the frequency of the interrupts. If the clock source is internal (peripheral bus), the period of the timer interrupt is given by
Period = [(PRx + 1) Tpb (TMR_Prescalar_value)], where Tpb is the period of the peripheral bus.
This means that if we want to specify the frequency of the interrupt, we need to determine PRx. Note that PRx needs to be less that 2^16 - 1 if using only 1 timer or less than 2^32 - 1 if combining two timers. See example below.
The peripheral library offers a number of function calls to simplify setting up and using the Timers. The declarations of these functions can be found in pic32-libs/include/peripheral/timer.h
Each Timer X (ie replace X with the timer number you wish to use) has the following functions:
- OpenTimerX(config, period): where config is a value selected by using | with the constants given in the header file. period is the desired PRx value which determines the roll-over // interrupt frequency.
- CloseTimerX(): this disables the timer
- ConfigIntTimerX(config): this function configures the interrupt based on config which is a value selected by using | with the constants given in the header file.
- EnableIntTX(): enables the interrupt
- DisableIntTX(): disables the interrupt
- SetPriorityIntX(priority): sets the priority of the interrupt
- ReadTimerX(): returns the value of the timer
- WriteTimerX(value): loads a given value to the timer
- ReadPeriodX(): returns the PRx value
- WritePeriodX(value): loads a given value to the PRx register
Timers 2 and 3 as well as 4 and 5 can be combined to create 32-bit timers. If using a 32-bit timer, replace the X in the functions above with 23 or 45.
This example is going to toggle 2 LEDs at a given frequency using interrupts with timer 2. We are going to use the functions OpenTimer2() and ConfigIntTimer2() to initialize the timer and interrupts. You will see in the example code that there is also a function INTEnableSystemMultiVectoredInt() which is used to set up multi-vectored interrupts (whatever that means).