Microprocessor Fundamentals. Topic 7 Timing: the Timer/Counter

Transcription

1 Microprocessor Fundamentals Topic 7 Timing: the Timer/Counter

2 Objectives Examine the Timer/Counter Register: TCNT0 Examine the Timer/Counter Control Register: TCCR0 Write a time delay for the previous program example: Light Chaser Use the Timer/Counter Design a ½ sec. delay for lights on/off Assume a 16 MHz clock 2/23/2012 2

3 Timer/Counter So, we have: The Light Chaser Program Must add a time delay for the program: Use the Timer/Counter Design a ½ sec. delay for lights on/off Assume a 16 MHz clock We need to look up the Timer/Counter 2/23/2012 3

4 Timer/Counter Open the ATmega128 Manual Click on the 8-Bit Timer/Counter 0 2/23/2012 4

5 Timer/Counter 8-Bit Timer/Counter Click on the 8-Bit Timer/Counter Registers 2/23/2012 5

6 TCCR0 The Timer/Counter Control Register We are most concerned with bits 2, 1, and 0. They control the clock source to be used by the counter 2/23/2012 6

7 TCCR0 The Timer/Counter Control Register We are most concerned with bits 2, 1, and 0. They control the clock source to be used by the counter We can turn off the counter We can use the system clock or We can use a slower signal based on the system clock 2/23/2012 7

8 TCCR0 The Timer/Counter Control Register We are initializing the Timer/Counter so that it will increment at every clock pulse. If the system clock was 10 MHz: If we used the system clock, the Timer/Counter would increment every 100ns 2/23/2012 8

9 TCCR0 The Timer/Counter Control Register We are initializing the Timer/Counter so that it will increment at every clock pulse. If the system clock was 10 MHz: If we used the system clock, the Timer/Counter would increment every 100ns If we used a prescaler of 1024 (bits 2-0 = 111), the Timer/Counter would increment every 100 us 2/23/2012 9

10 TCCR0 The Timer/Counter Control Register The real question: How long must our delay be? The Timer/Counter (an 8-bit register) can count from 0 to 255. With a 10 MHz clock and no prescaler, the Timer/Counter can count from 1 to 255 in 100ns to 25.5 us With a 1024 prescaler, it could count from 1 to 255 in 100us to 25.5 ms 2/23/

11 TCCR0 The Timer/Counter Control Register If we need a delay of 10 us, don t use a prescaler. The real question: How long must our delay be? The Timer/Counter (an 8-bit register) can count from 0 to 255. With a 10 MHz clock and no prescaler, the Timer/Counter can count from 1 to 255 in 100ns to 25.5 us With a 1024 prescaler, it could count from 1 to 255 in 100us to 25.5 ms 2/23/

12 TCCR0 The Timer/Counter Control Register If we need a delay of 10 us, don t use a prescaler. If we need a delay of 10ms, use the 1024 prescaler The real question: How long must our delay be? The Timer/Counter (an 8-bit register) can count from 0 to 255. With a 10 MHz clock and no prescaler, the Timer/Counter can count from 1 to 255 in 100ns to 25.5 us With a 1024 prescaler, it could count from 1 to 255 in 100us to 25.5 ms 2/23/

13 Example: Write the initialization routine and the timing loop to form a delay of 10ms (assume a 10 MHz clock) 2/23/

14 Example: Write the initialization routine and the timing loop to form a delay of 10ms (assume a 10 MHz clock) A 10 MHz clock will take 100ns/cycle A 1024 prescaler means we will increment the TC every 100us To count from 0 to 10 ms and incrementing TC every 100 us/count means we have to count from 0 to 10ms/100us 1 = = = 63 X We subtract 1 because we are counting from 0, not 1 2/23/

15 Example: Write the initialization routine and the timing loop to form a delay of 10ms (assume a 10 MHz clock) ;========================================================.def delayreg = r17.def TCValue = r18 ;name registers ;=========================== Init: ldi delayreg,0x07 ;set initial parameters for counter out TCCR0,delayReg ;prescaler = 1024 (ATmega128) ;============================ TDelay: ldi delayreg,0 ;initialize timer/counter 0 out TCNT0,delayReg Here s the program. I used a prescaler of 1024 to reduce the counter s clock from 10 MHz to 10 khz (111 in bits 2, 1, 0) TLoop: in TCValue,TCNT0 ; read timer/counter value cpi TCValue,0x63 ; is it $63? brne Tloop ; if not $63, read it again (until $63) done ;================================= 2/23/

16 Example: Write the initialization routine and the timing loop to form a delay of 10ms (assume a 10 MHz clock) ;========================================================.def delayreg = r17.def TCValue = r18 ;name registers ;=========================== Init: ldi delayreg,0x07 ;set initial parameters for counter out TCCR0,delayReg ;prescaler = 1024 (ATmega128) ;============================ TDelay: ldi delayreg,0 ;initialize timer/counter 0 out TCNT0,delayReg TLoop: in TCValue,TCNT0 ; read timer/counter value cpi TCValue,0x63 ; is it $63? brne Tloop ; if not $63, read it again (until $63) Here s the program. I used a prescaler of 1024 to reduce the counter s clock from 10 MHz to 10 khz (111 in bits 2, 1, 0) Then load the TC with 0 to initialize the count (it starts on its own) done ;================================= 2/23/

17 Example: Write the initialization routine and the timing loop to form a delay of 10ms (assume a 10 MHz clock) ;========================================================.def delayreg = r17.def TCValue = r18 ;name registers ;=========================== Init: ldi delayreg,0x07 ;set initial parameters for counter out TCCR0,delayReg ;prescaler = 1024 (ATmega128) ;============================ TDelay: ldi delayreg,0 ;initialize timer/counter 0 out TCNT0,delayReg TLoop: in TCValue,TCNT0 ; read timer/counter value cpi TCValue,0x63 ; is it $63? brne Tloop ; if not $63, read it again (until $63) done ;================================= Here s the program. I used a prescaler of 1024 to reduce the counter s clock from 10 MHz to 10 khz (111 in bits 2, 1, 0) Then load the TC with 0 to initialize the count (it starts on its own) Then we continuously read the TC (TCNT0) until its value is decimal 99 (or hex 63) if it does not have 63 in it yet, go back and read it again 2/23/

18 Exercise: Write the initialization routine and the timing loop to form a delay of 15ms (assume a 16 MHz clock) 2/23/

19 A Solution: Write the initialization routine and the timing loop to form a delay of 15ms (assume a 16 MHz clock) ;========================================================.def delayreg = r17.def TCValue = r18 ;name registers ;=========================== Init: ldi delayreg,0x07 ;set initial parameters for counter out TCCR0,delayReg ;prescaler = 1024 (ATmega128) ;============================ TDelay: ldi delayreg,0 ;initialize timer/counter 0 out TCNT0,delayReg TLoop: in TCValue,TCNT0 ; read timer/counter value cpi TCValue,0xEF ; is it $EF? brne Tloop ; if not $63, read it again (until $EF) done ;================================= Here s the program. I used a prescaler of 1024 again - to reduce the counter s clock from 16 MHz to 16 khz (111 in bits 2, 1, 0) Then load the TC with 0 to initialize the count (it starts on its own) Then we continuously read the TC (TCNT0) until its value is decimal 239 (or hex EF) if it does not have EF in it yet, go back and read it again 2/23/

20 Longer Delays: What if we need a longer delay? Example: ½ sec (500ms) or 1 sec (1000ms) If we assume a 10 MHz clock, the prescaler will help us get down to 10 khz Timer/Counter increments every 100 us Timer/Counter must count to: 5,000 for ½ sec 10,000 for 1 sec 2/23/

21 Longer Delays: What if we need a longer delay? Example: ½ sec (500ms) or 1 sec (1000ms) If we assume a 10 MHz clock, the prescaler will help us get down to 10 khz Timer/Counter increments every 100 us Timer/Counter must count to: 5,000 for ½ sec 10,000 for 1 sec 8 bit counters can only count to 255 2/23/

22 Longer Delays: To count to ½ sec (500ms) or 1 sec (1000ms): We need a nested loop Execute the delay loop more than once For example: Assume a 10 MHz clock, a prescaler of 1024, and a count in the Timer/Counter to 255 (0 to 255 is a count of 256) The TC increments every 100 us Once through the delay loop is 25.6 ms For a ½ sec delay, we would need to execute the delay loop 500ms/25.6ms = 19.5 times (19 or 20 whichever is closer) For a 1 sec delay: 1000ms/25.6ms = 39 times 2/23/

23 Longer Delays: Now, let s add the longer delay to the Light Chaser program. 2/23/

24 Enter Program Original Program Statements New registers Timer/Counter Initialization Delay Instructions inserted to remove the blank caused by the C bit (see previous topic) ;======================================================== : documentation ;=========================== ; Declarations.device ATmega128.def lights = r16 ;name registers.def delayreg = r17.def counter = r18.def CntValue = r19 ;=========================== Init: ldi lights,0b ;setup inputs and outputs for PortB out DDRB,lights ldi lights,0b ;set initial values for outputs out PortB,lights ldi delayreg,0x07 ;set initial parameters for counter out TCCR0,delayReg ;prescaler = 1024 (ATmega128) ;============================ ;Start Program Start: cpi lights,0 brne Next ldi lights,0b ;reset values for outputs Next: out PortB,lights ; sends data to Port B (1 lights LED) ror lights ; rotate bits once to the left ;================================= ;Time Delay TDelay: ldi counter,31 ;set initial # of times through delay loop ; 32 =.5 sec (64 = 1s) TLoop2: ldi delayreg,0 ;initialize timer/counter 0 out TCNT0,delayReg TLoop: in CntValue,TCNT0 ; read timer/counter value cpi CntValue,255 ; is it 0? brne TLoop ; if not 0, read it again (until 0) dec counter ; counting down to 0 for.5 sec brne TLoop2 rjmp Start 2/23/

25 Lights Now the LEDs should appear as shown when we download this to the AVR board: and then repeat 2/23/

26 Development Process If we have followed the development process, we have: Gathered and verified the requirements Laid out the plan (flowcharted) Chosen the target processor Written the program Assembled the program Now we have to simulate, but we have a new problem 2/23/

27 Development Process If we single step through the program as before, it will take a long time to go through the delays Fortunately, we have other options Set break points Run the program Let s start with the simplest: We get to the instruction 2/23/

28 Simulation Open AVR Studio, Assemble the program Best place to set a breakpoint is after the delay loop has finished executing 2/23/

29 Simulation Open AVR Studio, Assemble the program Best place to set a breakpoint is after the delay loop has finished executing To set the breakpoint: place the cursor at the command and click on the hand: 2/23/

30 Simulation Open AVR Studio, Assemble the program Best place to set a breakpoint is after the delay loop has finished executing To set the breakpoint: place the cursor at the command and click on the hand: 2/23/

31 Simulation To start debugging, click Debug on the Debug Menu or click on the (Hidden by menu) 2/23/

32 Simulation To start, click on the Run button 2/23/

33 Simulation To simulation will run to the breakpoint and stop All affected registers will be updated 2/23/

34 Simulation This will show us r18 (counter) counting down (it will take awhile) Reset the program and change the breakpoint to the rjmp instruction 2/23/

35 Simulation Each time you hit the Run button, you ll see the I/O registers get updated Now we should download it to the board and test it 2/23/

36 Summary We: Examined the Timer/Counter Register: TCNT0 Examined the Timer/Counter Control Register: TCCR0 Wrote a time delay for the previous program example: Light Chaser Used the Timer/Counter Designed a ½ sec. delay for lights on/off 2/23/