Overview of Lecture

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Overview of Lecture

• Timer functions
• Output-compare function
• Pulse/square-wave using output-compare function
• Programming example

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(No Transcript)
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HC11 Timer Function

• Many application requires dedicated timer
  functions
• Input-capture function to measure duration of
  pulse or period of square wave
• Input-capture function to measure duty cycle
  periodic waveform
• Output-compare function to create time delay
• Output-compare function to generate a pulse or
  square waveform
• Pulse accumulator to count number of events
• Time of day tracking
• Periodic interrupt generation to remind processor
  to perform routine tasks
• Waveform generation

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HC11 Timer Function

• HC11 includes sophisticated timer functions
• Free-running main timer
• Input-capture functions
• Output-compare functions
• Real-time Interrupt (RTI)
• Pulse accumulator

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Free-running main timer

• Main timer (TCNT) clocked by output of a
  four-stage pre-scaler (divided by 1, 4, 8 and
  16 and driven by E clock signal)
• Timer is cleared to 0 during a reset and is a
  read only register
• When count changes from FFFF to 0000, timer
  overflow flag (TOF) bit in timer flag register 2
  (TFLG2) is set
• Timer overflow flag can be cleared by writing a 1
  to it
• Interrupt is enabled by setting timer overflow
  interrupt enable bit (TOI) of timer interrupt
  mask register 2 (TMSK2)
• Pre-scale factor is for timer is selected by bits
  0 and 1 of TMSK2
• Timer counter (TCNT) register is meant to be read
  using a double-byte read instruction, e.g. LDD,
  LDX

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Free-running main timer cont

• TCNT can be used to keep track of time
• TCNT can also be used for time delays as well
• Example
• 10-ms delay at 2-MHz clock rate is equivalent to
  20K clock cycles
• By adding 20K to the current value of TCNT and
  waiting until this value is smaller than
  incrementing TCNT, a 10ms delay is created
• TCNT EQU 0E Values defined
• LDX 1000
• LDD TCNT,X
• ADDD 20000
• Loop CPD TCNT,X Compare Acc D with 16-bit
  value at address
• BGT Loop Branch if greater than zero

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Input-capture functions

• HC11 timer system has three input-capture
  channels
• Each input-capture channel includes
• a 16-bit input-capture register
• Input-edge detection logic
• Interrupt-generation logic
• Physical time is represented by the count in
  16-bit free-running timer counter TCNT
• Input-capture function is used to record time
  when an external event occurs
• When a selected edge is detected at input pin,
  contents of TCNT are latched into three
  input-capture registers
• Captured 16-bit timer values is stored in
  input-capture register these are
• TIC1, TIC2 and TIC3 located at 1010-1011,
  1012-1013 and 1014-1015 respectively

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Input-capture functions cont

• Pins PA0, PA1 and PA2 are used as input-capture
  channels IC1, IC2 and IC3 respectively
• User can select signal edge (rising, falling or
  both) to capture by programming 6-bit TCTL2
  register located at address 1021

Figure 7.2 Input-capture function
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Input-capture functions cont

• Example
• PA0 pin used as input-capture channel IC3. To
  capture rising edge, the edge select bits (bits 1
  and 0 of TCTL2) of the channel IC3 must be set to
  01. Following instruction will set up TCTL2
  capture time of a rising edge on PA0.
• LDX 1000
• BCLR TCTL2, X 02 clear bit 1 0 without
  affecting other bits
• BSET TCTL2, X 01 set bit 0 to 1 without
  affecting other bits

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Output-Compare Function

• Pins PA3-PA6 are used as output-compare channels

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Output-Compare Function Cont

• HC11 has 5 output-compare functions
• Control level on output pins PA7-PA3
• Each output-compare function consists of
• A 16-bit comparator
• A 16-bit compare register TOCx
• An output action pin OCx
• An interrupt request circuit
• A forced-compare function FOCx
• Control logic

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Operation of Output-Compare Function

• Main application of output-compare function
• To toggle a signal
• Turn on a switch etc
• Technique to use output-compare function
• Makes a copy of the current contents of timer
  (TCNT)
• Adds to this copy a value equal to the desired
  delay
• Stores the sum into an output-compare register
• User specifies action to be activated on selected
  output-compare OCx pin by programming TCTL1
  register

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Operation of Output-Compare Function cont

• TCTL1 register has the following format
• OMx OLx
• --------------------------------------------------
  -----------------------------
• 0 0 OCx does not affect pin
• 0 1 Toggle OCx pin on successful compare
• 1 0 Clear OCx on successful compare
• 1 1 Set OCx pin on successful compare
• --------------------------------------------------
  -----------------------------
• Compare value at TCNT
• Set corresponding flag bit in TFLG1 register
• Output-compare interrupt enabled by TMSK1 register

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Programming Example

• Generate an active high 1 KHz digital waveform
  with 40 duty cycle from output-compare pin OC2.
  Use the polling method to check the success of
  the output compare. The frequency of the E clock
  is 2MHz, and the pre-scale factor to the
  free-running timer is 1.
• An active high 1KHz waveform with 40 duty cycle
  looks like this

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Duty Cycle

• Duty cycle is the percent of time that the signal
  is high within a period of waveform
• Example
• Duty cycle T1/T100

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C language version of the program
include lthc11.hgt include ltstdio.hgt main()
PORTA 0x40 //set OC2 pin high TCTL1
0x40 //select toggle as OC2 pin action
TOC2TCNT800 //start OC2 compare with 800
cycles //For 2Mhz E clock, 800 and 1200 clock
cycles correspond to delays of 400µs and //600µs
respectively TFLG10x40 //clear OC2F
flag while(1) while(!(TFLG1 0x40)) //wait
unit 800 cycles is over TFLG10x40 //clear
OC2F flag TOC2TOC21200 //start next
operation while(!(TFLG1 0x40)) //wait until
1200 cycles FLG10x40 TOC2TOC2800 re
turn 0
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Tutorial 6-7