Chapter 5 - MARV's Heartbeat: Oscillators and Timers

Oscillators

• Every digital device requires a clock source to synchronize operations.
• This chapter explores clock sources for PIC18 devices and the functioning of timers/counters.
• An oscillator consists of an inverting amplifier and a filter (tuned circuit or crystal) in a positive feedback loop.
• Barkhausen criteria for sustained oscillations:
  • Loop gain must exceed unity at the resonant frequency.
  • Phase shift around the loop must be zero or an integer multiple of 2π radians.
• The inverting amplifier provides voltage gain and a 180-degree phase shift.
• The tuned circuit or crystal provides another 180-degree phase shift and determines the oscillator's frequency.

Clock Sources

• The PIC18F45K22 can use six primary clock sources:
  • Low-frequency (e.g., 31 kHz internal RC oscillator)
  • High-frequency (e.g., up to 64 MHz crystal oscillators)
• Choose the lowest oscillator speed that meets system requirements to minimize power consumption.
• FOSC
• A secondary internal clock source (32.768 kHz) is shared with Timers 1, 3, and 5.
• System clock select bits (SCS

External RC Oscillator (RC)

• A resistor-capacitor combination provides a low-cost external oscillator.
• Microchip Developer Help documentation explains the design process.

External Low-Power Crystal Oscillator (LP)

• Uses a low-power crystal driven by the internal inverting amplifier at its lowest gain setting.
• Designed for a low-frequency 32.768 kHz tuning-fork-type crystal.
• Dividing the frequency by 2^{15} = 32768 yields a 1 Hz signal, commonly used in time-keeping applications (wristwatch crystals).
• Offers the lowest current consumption among the external crystal oscillator modes.

External Crystal or Resonator Oscillator (XP)

• Accommodates crystals or resonators with a midrange frequency (0.5 MHz to 4 MHz).
• The internal inverting amplifier is set at an intermediate gain.
• Operates at medium power consumption.

External High-Speed Crystal or Resonator (HS)

• The PIC18F45K22 can operate from a clock source up to 64 MHz.
• For frequencies above 4 MHz, select the external high-speed oscillator mode.
• The internal inverting amplifier gain must be set at medium for frequencies between 4 and 16 MHz and at high for frequencies above 16 MHz.
• Power consumption is high in this mode.

External Clock (EC)

• Uses a clock source generated by another device, connected to the device's OSC1 pin.
• Has three power modes:
  • Low power: Clock signals below 0.5 MHz
  • Medium power: Clock signals between 0.5 and 16 MHz
  • High power: Clock signals exceeding 16 MHz

Internal RC Oscillator (INTOSC)

• The internal RC oscillator module has three independent oscillators:
  • High-frequency oscillator (HFINTOSC): Operates up to 16 MHz
  • Medium-frequency oscillator (MFINTOSC): Operates up to 500 kHz
  • Low-frequency oscillator: Operates at 31.25 kHz
• Internal oscillator frequency select bits IRCF
• MFINTOSC and HFINTOSC can be tuned via the OSCTUNE register.
• The internal oscillator can be output on port pins to drive external digital devices; this is controlled by configuration bits.

Oscillator Start-Up Delay

• Crystal oscillators take time to stabilize.
• Executing firmware before stabilization can cause timing errors.
• PIC18s have an oscillator start-up timer (OST) that, if enabled, counts 1024 oscillations after a power-on reset and keeps the device reset during this period.
• RC oscillators are stable when switched on.
• The PIC can be configured to use the internal oscillator until an external crystal or resonator oscillator stabilizes.
• Two-speed start-up advantages:
  • Faster start-up
  • Reduced power consumption, as the high-speed oscillator might not need to start for small events.
• Clock-switching can be implemented in firmware to save power by using different clock sources for different tasks.

Timers

• A timer on a microcontroller is a register or register pair that counts on a regular clock.
• If the period of a count is known, the duration for a specific number of counts is also known - this can be used to time events.

Introduction to Timers

• Timers are commonly used to implement delays.
• PIC18s have 8-bit and 16-bit timers.
  • 8-bit timer: Counts up to 256 (0 to 255).
  • 16-bit timer: Counts up to 65,536. Implemented as two 8-bit registers (high and low byte).
• Timer overflow: When the timer restarts from zero after reaching its maximum value; this generates an overflow pulse.
• Timers derive their time base from the instruction clock. If no pre- or postscalers are used, they increment at FOSC/4.
• If FOSC is 4 MHz, the instruction cycle TCY is 1 µs.
• An 8-bit timer will take 256 µs to count from zero to zero again.
• A 16-bit timer will take around 65.5 ms to count from zero to zero again at this clock frequency.
• If the timing requirement is more than the timer period, a number of timer periods may be implemented back-to-back.

Pre- and Postscalers

• The timer period can be expanded through pre- and postscalers.

• The prescaler is a divider that divides the instruction clock. If the prescaler is set at 1:4, its increment period would be four times TCY.

• The postscaler is another adjustable counter that counts the number of timer overflow pulses.
  Figure 5.2 Pre and post scalers for Timer 2 as shown in the datasheet of the PIC18F45K22.

• The timer interrupt flag TMRxIF will only be set once the number of timer overflow pulses that was set in the postscaler has been counted.

• Example:

  • Use an 8-bit timer to implement a 1 s delay for FOSC = 4 MHz.
  • A prescale value of 1:4 will result in a timer period of around 1 ms (4 x 256 µs).
  • 1000 timer periods would be required to create the 1 s delay.
  • Without pre- or postscalers, approximately 4000 timer periods would be required to create 1 s.

Period Registers

• Some timers have a period register that can be programmed with a specific value.
• A comparator compares the value of the timer with the value in the period register after each count.
• When the values match, the timer is reset (restarts from 0).
• A timer period when using a period register value of 249 will be 4 * 250 µs * 4 * 250 interrupts = 1.000 s.
• The period register always has the same number of bits as the timer.

Timers as Counters

• Timers can be incremented from a regular clock source or by an external trigger.
• If an external irregular trigger is used to increment the timer, it counts events rather than acting as a timer.
• Timers are frequently referred to as timers/counters.

Timers on the PIC18F45K22

• The PIC18F45K22 has three 8-bit timers and four 16-bit timers.
• Timer 0:
  • Can be configured as an 8- or 16-bit timer or counter.
  • To use it as a counter, the trigger must be connected to the T0CKI pin.
  • Has an 8-bit prescaler with values in powers of 2 from 1:1 to 1:256.
• Timers 1, 3, and 5:
  • 16-bit timers/counters (TMRxH:TMRxL).
  • The edge of the clock input on which the increment should occur can be selected.
  • Have several gate sources.
  • Have a 2-bit prescaler to allow 1:1 to 1:8 instruction clock divisions.
  • Do not have a postscaler.
  • Timer 1 can also be driven from a secondary low-power 32.768 kHz oscillator for time-keeping applications.
  • The Capture/Compare/Pulse Width Modulation (CCP) peripheral uses Timers 1, 3 and 5 for its time-base.
    *Timers and counters register pair in the CCP module thus acts as a 16-bit period register for Timers 1, 3 and 5.
• Timers 2, 4, and 6:
  • 8-bit timers with pre- and postscalers and a built-in period register.
  • Handy for implementing precise timing applications (e.g., delays).

NIBBLE on Timing and Timers

• This Chapter's NIBBLE assignments cover timers, counters and oscillators.

The basics you should gain from this NIBBLE

Oscillators

• An understanding of
  • the various primary clock modes of the PIC18F45K22.
  • the difference between an internal and an external clock source, and the ways in which these sources can be implemented for the PIC18F45K22
  • how to set up the internal oscillator frequency

Timers and counters

• An understanding of
  • the nature and operation of µC timers.
  • the difference between 8-bit and 16-bit timers.
  • the clock source employed by the timers.
  • the concept and operation of pre- and postscalers.
  • the concept and operation of a period register.
  • how timers can be used to count events, either synchronously or asynchronously.

Code Example: Timer 2 Delay

• Flashes an LED on RD0 three times and then hangs in an infinite loop.
• Implements a 1-second delay using Timer 2 with a period register of 199, FOSC = 500 kHz, a prescaler of 1:4, and a postscaler of 1:1.
• The timer period is 4 * (199+1) * TCY * 1 = 4 * 200 * 8 µs * 1 = 6.4 ms.
• To implement a delay of 1 second, 1000 ms / 6.4 ms = 156.25 timer interrupts must be allowed to occur, and since only integer values of the timer interrupts can be counted, the value is rounded to the nearest integer to 156.
• Relevant code sections:
  • Timer 2 control register is loaded with the value for the prescaler, postscaler, and TMR2ON bit.
  • The period register (PR2) is loaded with 199.
  • Interrupts are initialized and enabled (PEIE, GIE, TMR2IE bits are set).
  • The timer is cleared (TMR2 is cleared) and started (TMR2ON is set).
  • A wait loop is implemented until the required number of interrupts has occurred (DelayFlag is set).
  • In the ISR, the Count register is incremented, and the value is compared to 156. When Count reaches 156, DelayFlag is set.
  • Once the delay flag is set, the timer and interrupt are disabled, and the Count register is cleared.

Code Example: Simple Timer 2 Example

• Illustrates that 10 counts are executed for a period register that is preloaded with a value of 9.
• Shows the number of counts taken for the branches to and from the ISR.

Additional Resources

• PICmicro MID-RANGE MCU FAMILY: Chapter 2 Oscillators
• Intel AP-155 Oscillators for Microcontrollers
• Microcontroller Oscillator Circuit Design Considerations
• AN826 Crystal Oscillator Basics and Crystal Selection for rfPIC and PIC Devices
• AN849 Basic PIC Oscillator Design
• AN943 Practical PIC Oscillator Analysis and Design
• AN949 Making Your Oscillator Work