1 / 59

Week #6 Timing Generation and Measurement

Week #6 Timing Generation and Measurement. ENG3640 Microcomputer Interfacing. Topics. Timer Module Input Capture Output Compare Pulse Accumulator Pulse Width Modulation. Resources. Huang, Chapter 8, Sections

inari
Download Presentation

Week #6 Timing Generation and Measurement

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Week #6 Timing Generation and Measurement ENG3640 Microcomputer Interfacing

  2. Topics • Timer Module • Input Capture • Output Compare • Pulse Accumulator • Pulse Width Modulation ENG3640 Fall 2012

  3. Resources • Huang, Chapter 8, Sections • 8.2 Why are Timer Functions Important? • 8.3 Standard Timer Module • 8.4 Timer Counter Register • 8.5 Input Capture Function • 8.6 Output Compare • 8.8 Pulse Accumulator • 8.10 Pulse Width Modulation ENG3640 Fall 2012

  4. 1-KB SRAM 4-KB EEPROM 68HC812A4 Block Diagram CPU12 Port T Timer Module ENG3640 Fall 2012

  5. Introduction • Instruction timing (software) is indeterminate (on most processors) to achieve desired level of precision • The timer module is used to handle interfacing with time-precise events • pulse measurement • accelerometer • pulse counting • shaft rotation • pulse modulation • limb actuator ENG3640 Fall 2012

  6. Timer Block Diagram Very Complicated! So we need to divide and conquer to understand the functionality of each sub-module. ENG3640 Fall 2012

  7. Register Listing • 25 registers • 10 16 bit HC12 ENG3640 Fall 2012

  8. Interrupt Vectors ENG3640 Fall 2012

  9. Basic Workings • The module clock, which is the system clock, at 8Mhz, increments a 16 bit counter called TCNT. • Eight channels use this as a reference. • 16 bit register • 16 bit comparator • Eight associated input/output pins for reading signals or creating waveforms. • Registers include channel and timer control registers ENG3640 Fall 2012

  10. (I) Output Compare Module • The OC pin is an output of the MCU, hence can be used to control an external device. • The output compare module can be used to • Create square waves • Generate pulses • Implement time delays • Execute periodic interrupts • Pulse Width Modulation ENG3640 Fall 2012

  11. Basic Components of Output Compare TCNT 15 0 • An output compare event occurs when either the 16-bit TCNT matches the 16-bit OC register or software writes a 1 to the FOC bit. • The OMn, OLn bits specify what effect the output compare event will have on the output pin. • The module is armed when the mask bit is 1. • The flag is set when the event occurs OC reg OCn mask flag FOC OMn OLn ENG3640 Fall 2012

  12. Output Compare Actions • Each channel can be programmed to affect the output pin to toggle, go high, go low or do nothing OM0 OL0 ENG3640 Fall 2012

  13. Basic Principles of Output Compare • Two or three actions result from an Output Compare  (TCNT = OC REG) • The OCn output bit changes. • The output compare flag is set • An interrupt is requested (only when mask is 1) TCNT OC reg ENG3640 Fall 2012

  14. Output Compare: Functional Diagram 16-bit Comparator OMn:OLn ENG3640 Fall 2012

  15. Compare to TCNT OCxF ENG3640 Fall 2012

  16. A Simple Delay Using Output Compare • Let fixed be the number of cycles you wish to wait (i.e the delay). • The steps to create the delay are: • Read the current 16-bit TCNT • Calculate TCNT + fixed • Set the 16-bit output compare register to TCNT + fixed • Clear the output compare flag • Wait for the output compare flag to be set (polling or interrupt) ENG3640 Fall 2012

  17. Output Compare: PWM • Pulse Width Modulation or Variable Duty Cycle Actuator is a technique used to control motors by controlling the amount of power supplied (varying the duty cycle) Small Middle Large ENG3640 Fall 2012

  18. Enabling/Using the OC Module • The first step to use the Output Compare Module is to set the bit ``TEN” in the TSCR to “1”. This allows the TCNT to increment. • TSWAI: When set to ‘1’ the TCNT stops in wait mode. • TSBCK: When set to `1` the TCNT stops in background mode. • TFFCA: (Timer fast flag clear all) $0086 TSCR TEN TSWAI TSBCK TFFCA 0 0 0 0 ENG3640 Fall 2012

  19. Enabling/Using the OC Module • The second step is to set the prescaler in TMSK2 • This determines the rate by which TCNT counts PR2 PR1 PR0 ENG3640 Fall 2012

  20. Enabling/Using the OC Module • The third step is to select the pin on the timer PT0 .. PT7 as an output compare by placing a `1` in the corresponding bit of the TIOS register. • There is also a direction register DDRT, for Port T, which means we should set the corresponding bits for the output compare. • Make sure also to specify the action (toggle, set or reset) determined by Omx:OLx $0080 TIOS IOS7 IOS6 IOS5 IOS4 IOS3 IOS2 IOS1 IOS0 ENG3640 Fall 2012

  21. $008E TFLG1 C7F C6F C5F C4F C3F C2F C1F C0F Enabling/Using the OC Module • The fourth step is to write time you want event to occur to TCn register • To have event occur on Pin 4 when TCNT == 0x000: TC4 = 0x000; • To have next event occur on Pin 4 (500 cycles later): TC4 = TC4 + 500; • When TCNT == TCn, the specified action will occur, and flag Cfn will be set. • To clear the flag, write a 1 to the bit you want to clear ENG3640 Fall 2012

  22. TMSK1 C7I C6I C5I C4I C3I C2I C1I C0I Enabling/Using the OC Module The fifth step is to arm or disarm the output compare interrupts by initializing the TMSK1 register. $008C ENG3640 Fall 2012

  23. Square Wave Generation: Example • This example generates a 50% duty cycle square wave using output compare. • The output is high for Period cycles, then low for Period cycles. • Toggle mode is used to create the 50% duty cycle square wave. • In this mode, the output compare pin is toggled whenever the output compare latch matches TCNT. 6812 PT3/OC3 ENG3640 Fall 2012

  24. Square Wave Generation: Example ; M68HC812A4. (Initialization routine) Init sei ; disable interrupt (make atomic) bset TIOS, $08 ; Selecting pin 3 as OC bset DDRT, $08 ; PT3 output movb #$80, TSCR ; enable timer (bit “TEN”) movb #$32, TMSK2 ; 500 ns clk (prescalar) bset TMSK1, $08 ; Arm OC3 bset TCTL2, $40 ; OL3 = 1 bclr TCTL2, $80 ; OM3 = 0 (Toggle output line) movb #$08, TFLG1 ; clear Flag for OC3 (IMPORTANT) ldd TCNT ; current time addd #50 ; in 25 micro seconds std TC3 cli ; enable interrupts rts ENG3640 Fall 2012

  25. Square Wave Generation: ISR ; MC68HC812A4 OC3Han movb #$08, TFLG1 ; Ack [4] ldd TC3 [3] addd Period ; next [3] std TC3 [2] rti [8] org $FFE8 ; timer channel 3 fdb OC3Han ENG3640 Fall 2012

  26. Basic Components of Input Capture TCNT 15 0 • The EDGnA, EDGnB specify whether the rising or falling, or both will trigger a capture event. • The module is armed when the mask bit is 1 (i.e. an interrupt will be requested on a capture event). • The flag bit is set when a capture event occurs. IC reg ICn mask flag EdgnB EdgnA ENG3640 Fall 2012

  27. Basic Principles of Input Capture • Two or three actions result from a capture event • The current TCNT value is copied into the input capture register. • The input capture flag is set • An interrupt is requested (only when mask is 1) TCNT IC reg ENG3640 Fall 2012

  28. Input Capture: Functional Block Diagram TCNT TCn EDGnA:EDGnB ENG3640 Fall 2012

  29. Input Capture: Control Registers • Each channel can specify which edge to capture count on EDG0B EDG0A ENG3640 Fall 2012

  30. Input Capture Actions ENG3640 Fall 2012

  31. Input Capture: Applications • The input capture can be used in the following applications • Perform a rising edge capture, then a falling edge capture, subtract the two measurements and get pulse width Pulse Width • The minimum pulse width for the input capture input is greater than two module clocks. ENG3640 Fall 2012

  32. Input Capture: Applications • The input capture can also be used in the following applications • Perform two rising edge input captures and subtract the two measurements to get period. • The subtraction of Second – First remains valid even if the TCNT overflows and wraps around. Period First Second ENG3640 Fall 2012

  33. Enabling/Using the Input Capture Module • The first step to use the Input Capture Module is to set the bit ``TEN” in the TSCR to “1”. This allows the TCNT to increment. • TSWAI: When set to ‘1’ the TCNT stops in wait mode. • TSBCK: When set to `1` the TCNT stops in background mode. • TFFCA: (Timer fast flag clear all) $0086 TSCR TEN TSWAI TSBCK TFFCA 0 0 0 0 ENG3640 Fall 2012

  34. Enabling/Using IC: Clock Source • The second step is to set the prescaler in TMSK2 Register. • The input to the timer module can be adjusted to produce different time bases (PR2, PR1, PR0) • TOI: Timer Overflow Interrupt Enable • TPU: Timer pull-up resistor enable (0  no pull-up on Port T input) • TDRB: Timer Drive Reduction (0  normal current) • TCRE: Timer counter reset enable (0  free running counter) PR2 PR1 PR0 ENG3640 Fall 2012

  35. Enabling/Using the IC Module • The third step is to select the pin on the timer PT0 .. PT7 as an input capture by placing a `0` in the corresponding bit of the TIOS register. • There is also a direction register DDRT, for Port T, which means we should clear the corresponding bits for the input capture inputs. • Make sure also to specify the active edge using the TCTL3 and TCTL4 (discussed earlier). $0080 TIOS IOS7 IOS6 IOS5 IOS4 IOS3 IOS2 IOS1 IOS0 ENG3640 Fall 2012

  36. $008E TMSK1 TFLG1 C7I C7F C6I C6F C5I C5F C4I C4F C3I C3F C2I C2F C1I C1F C0I C0F Enabling/Using the IC Module The fourth step is to arm or disarm the input capture interrupts by initializing the TMSK1 register. $008C • Our software can determine if an input capture has occurred by reading the TFLG1 register. • The flags are cleared by writing a 1 to them. ENG3640 Fall 2012

  37. Interrupt Registers TOF • The timer overflow has a separate flag in TFLG2 ENG3640 Fall 2012

  38. Input Signal Conditioning • An op-amp can serve as a signal converter, transforming a sine wave into a square wave that can then be measured • Could be used for period measurement. ENG3640 Fall 2012

  39. Period Measurement: Initialization external signal to PT1/IC1 Period rmb 2 ; units 500 ns First rmb 2 ; TCNT at first edge Done rmb 1 ; set each rising Init sei ; make atomic bset TIOS, $02 ; PT1 = input capture bset DDRT, $02 ; PT1 is input movb #$80, TSCR ; enable timer (bit “TEN”) movb #$32, TMSK2 ; 500 ns clk ldaa TCTL4 ; read old value of programmable edge reg anda #$F3 ; Clear EDG1B = 0 oraa #$04 ; Set EDG1A = 1 staa TCTL4 ; Program PT1 on rise of IC1 movw TCNT, FIRST ; init global var clr Done movb #$02, TFLG1 ; clear IC1F bset TMSK1, $02 ; ARM IC1F cli ; enable interrupts rts ENG3640 Fall 2012

  40. Period Measurement: ISR ; MC68HC812A4 IC1Han movb #$08, TFLG1 ; clear C3F [4] ldd TC1 [3] subd First [3] std Period [3] movw TC1, First [6] movb #$FF, Done [4] rti [8] org $FFEC ; timer channel 1 fdb IC1Han ENG3640 Fall 2012

  41. Smallest Period • To calculate the smallest period that can be handled by the assembly language input capture interrupt we have to find: • The time to process the interrupt • The time to execute the entire handler ENG3640 Fall 2012

  42. Optoelectronic Position/Motion Detection • In many cases the MCU uses input capture to monitor and control speed. • A common type of speed sensor is an encoder disk mounted on a shaft. • Disk is located between a slotted optical switch • When encoder disk rotates the switch outputs pulses • Eight pulses correspond to one disk revolution so pulse frequency is related to speed of revolution. ENG3640 Fall 2012

  43. Extra Slides ENG3640 Fall 2012

  44. Timer Module: Input Capture and Output Compare • Input Capture: “captures the time at which an external event occurs”. Can be used to: • Generate interrupts and • Measure period or pulse width • Measure Frequency • Output Compare: “can generate a periodic pulse with a programmable polarity, duration, and frequency”. Can be used to: • Create periodic interrupts, • Generate: Pulses, Square waves ENG3640 Fall 2012

  45. Issues: Overflow • If the period is 8192 us, the IC1 interrupts will be requested every 16,384 cycles (assuming we increment TCNT every 500ns.) • The difference between TIC1 latch values will be 16384 = $4000. • This subtraction remains valid even if the TCNT overflows and wraps around in between IC1 interrupts. • $2000 – $E000  $4000 • This method will not operate properly if the period is larger than 65,535 cycles, or 32,767 us. ENG3640 Fall 2012

  46. Input Frequency Measurement: Frequency Resolution • The frequency resolution, (delta f) is defined to be the smallest change in frequency that can be reliably measured by the system. • For the system to detect a change, the frequency must increase (or decrease) enough so that there is one more (or less) pulse during the fixed time interval. • Therefore, the frequency resolution is • delta f = 1/fixed time • So if we count pulses in 10-ms time interval, the frequency resolution is 100HZ. • The frequency would have to increase to 600 Hz or decrease to 400 Hz for the change to be detected. ENG3640 Fall 2012

  47. Input Pulse Measurement ENG3640 Fall 2012

  48. Frequency Measurement • The direct measurement of frequency involves counting input pulses for a fixed amount of time. • The basic idea is to use • Input capture to count pulses • Output compare to create the fixed time interval. ENG3640 Fall 2012

  49. Input Frequency Measurement • We could initialize Input Capture to interrupt on every rising edge of input signal. • During the input capture handler, we could increment a Counter (at the beginning of our fixed time interval, the Counter is initialized to zero). • Counting the number of inputs capture during the time interval, you can calculate input frequency f = counter/fixed time ENG3640 Fall 2012

  50. Output Compare 7 Config Registers • PT7/OC7 on the 6812 can be configured such that an output compare event on it will cause changes on some or all of the other output compare pins. Setting OC7Mx bit configures TIMPORT pin to be output. The value in OC7D  output pin when TC7 matches TCNT • Usage? • This can be used for synchronization, i.e. creating pulses that start together or end together. ENG3640 Fall 2012

More Related