Microcontroller Timers and Interrupts

Working with Time: Interrupts, Counters, and Timers

• Microcontrollers need to deal with time for external events, measuring time, and generating time-based activity
• Microcontrollers use timers, counters, and interrupts for this

Interrupts

• Interrupts cause the microcontroller to halt its normal program flow
• Interrupts execute a separate program called the interrupt service routine (ISR)
• Interrupts are hardware-initiated subroutine calls typically generated by I/O devices

Interrupts vs. Polling

• Advantages of interrupts include immediate response to I/O service requests and continued normal execution until I/O service is required
• Disadvantages of interrupts include coding complexity, extra hardware needs, and the processor I/O device needing to generate an interrupt request

General Hardware Structure for Interrupts

• Interrupt sources can be external, internal, maskable, and non-maskable
• Maskable interrupts can be enabled/disabled by setting/clearing bits
• Non-maskable interrupts can not be disabled
• Each interrupt has a flag (a bit) that is set when an interrupt occurs

16F84A Interrupt Structure

• Sources of interrupts for the 16F84A are external interrupt only, timer overflow interrupt, port B on change interrupt, and EEPROM write complete interrupt
• INTCON register and option register are interrupt related bits

INTCON Register

• INTCON register address is located at 0Bh and 8Bh
• GIE: Global Interrupt Enable bit enables or disables all unmasked interrupts
• EEIE: EE Write Complete Interrupt Enable bit enables or disables EE Write Complete interrupts
• TOIE: TMRO Overflow Interrupt Enable bit enables or disables the TMRO interrupt
• INTE: RB0/INT External Interrupt Enable bit enables or disables the RB0/INT external interrupt
• RBIE: RB Port Change Interrupt Enable bit enables or disables the RB port change interrupt
• TOIF: TMRO Overflow Interrupt Flag bit indicates TMRO register overflow
• INTF: RB0/INT External Interrupt Flag bit indicates RB0/INT external interrupt occurred
• RBIF: RB Port Change Interrupt Flag bit indicates state change on RB7:RB4 pins

Option Register (81H)

• RBPU bit enables or disables PORTB pull-ups
• INTEDG bit selects interrupt trigger edge on RB0/INT pin. 1 = rising edge, 0 = falling edge
• T0CS bit selects TMRO clock source, 1 = RA4/TOCKI pin, 0 = internal instruction cycle clock (CLKOUT)
• TOSE bit selects TMRO source edge; 1 = high-to-low, 0 = low-to-high
• PSA bit assigns prescader to the WDT or the Timer0 module. 1 = the WDT. 0 = the Timer0 module
• PS2:PS0 bits set prescaler rate

Operation

• To use interrupts
  • Start the interrupt service routine at 0x0004
  • Clear the flag of the used interrupt in the INTCON register if it is not cleared on reset
  • Enable the corresponding interrupt by setting its bit in INTCON register
  • Enable global interrupts by setting the GIE bit
  • End the interrupt subroutine with RETFIE instruction to resume program execution

Context Saving

• If the main program is to preserve the W register, save it temporarily at the beginning of the ISR then restore at the end of ISR
• If preserving a memory location like STATUS
  • Save it temporarily in memory at the ISR beginning
  • Restore the value at the end of ISR

Counters and Timers

• Digital counters can be built with flip-flops and can count up/down, reset, or load an initial value
• An overflow indicates when the most significant bit changes from 1 to 0
• Counters with a known clock frequency can be used as timers

Timer applications

• Measure the time between two events
• Measure the time between two pulses
• Measure a pulse duration
• Polling or interrupts can be used

Watchdog Timer

• A timer internal to the microcontroller and continually counts up
• Microcontroller resets if enabled and overflows
• Helps reset the microcontroller if a program fails or gets stuck
• The WDT timer is enabled/disabled by the WDTE bit in the configuration word
• It has its own internal RC oscillator with a nominal time-out period of 18 ms
• Extended through prescaler bits in the OPTION register up to 128x18 ms = 2.3 sec
• The WDT timer is cleared using the CLRWDT instruction

Sleep Mode

• An important way to save power and can be entered using the SLEEP instruction
• Halts microcontroller operation, switches off the oscillator
• WDT is cleared unless, if it remains enabled, it keeps running
• Ports retain setting and ports bits are set/cleared respectively
• To exit
  • An interrupt occurs
  • A WDT wake-up
  • External reset the MCLR pin

Summary

• Microcontrollers deal with time using timers and interrupts
• Interrupts require attention only when they occur, saving computational power
• Most interrupts are configurable
• Hardware timers can be a counter or a timer and are useful for measuring time