1. Timers and Interrupts
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2. Ripple counter with D-type flip-flops
Q* output is fed back to the D input on each D- type latch and to the clock input on the following latch.
Each D-type output flip-flops from high to low, halving the frequency each step along the chain.
Outputs Q0 to Q3 give a 4 bit counter from $0 to $F…
… and then to $o
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3. Timers and counters on ATmega128
ATmega128 has 2 8-bit counter/timers and bit counter/timers.
Each is programmable via control registers to perform a wide variety of different functions:
Count-up, count-down, time external events (input capture), provide timed output waveforms (PWM), etc.
And as a source of interrupts!
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4. Processor Interrupts
Interrupts are subroutine calls initiated not by an rcall command but by hardware signals. These hardware signals cause the processor to jump to interrupt service routines. At the end they return control to your program just where it was just before the interrupt occurred.
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5. The Need for Processor Interrupts
Up to now if you wanted to do something in a program as soon as a bit was set (key pressed, bit set in a register, voltage exceeded a given threshold,…) you had to keep reading the bit until it changed !
This is called Polling
This is clearly not an efficient way of doing things
CPU time is wasted watching for something to happen
This is why interrupts were introduced as a means of getting the processor’s attention on demand
Use the CPU for other things until whatever we were waiting for has happened
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6. ATmega128 Timers and Interrupts
There are 24 different sources of interrupts
There are timers on the ATMega128 that can be used to trigger an interrupt at fixed intervals
Interrupts can be triggered when certain hardware tasks are completed
Eight external inputs can be used to request an interrupt
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7. The ATmega128 Memory Map
The first 35 2-word addresses in flash program memory are reserved for interrupts: your program jumps to one of these addresses if an interrupt occurs.
A table of jump instructions is used to let the processor know where to execute code in case of an interrupt
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8. Interrupts mapping External interrupts ;Address;
$0000 jmp RESET ; Reset Handler
$0002 jmp EXT_INT ; IRQ0 Handler - PortD
$0004 jmp EXT_INT ; IRQ1 Handler - PortD
$0006 jmp EXT_INT ; IRQ2 Handler - PortD
$0008 jmp EXT_INT ; IRQ3 Handler - PortD
$000A jmp EXT_INT ; IRQ4 Handler - PortE
$000C jmp EXT_INT ; IRQ5 Handler - PortE
$000E jmp EXT_INT ; IRQ6 Handler - PortE
$0010 jmp EXT_INT ; IRQ7 Handler - PortE
$0012 jmp TIM2_COMP ; Timer2 Compare Handler
$0014 jmp TIM2_OVF ; Timer2 Overflow Handler
$0016 jmp TIM1_CAPT ; Timer1 Capture Handler
External interrupts
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9. Interrupts mapping $0018 jmp TIM1_COMPA ; Timer1 CompareA Handler
$001A jmp TIM1_COMPB ; Timer1 CompareB Handler
$001C jmp TIM1_OVF ; Timer1 Overflow Handler
$001E jmp TIM0_COMP ; Timer0 Compare Handler
$0020 jmp TIM0_OVF ; Timer0 Overflow Handler
$0022 jmp SPI_STC ; SPI Transfer Complete Handler
$0024 jmp UART_RXC ; UART0 RX Complete Handler
$0026 jmp UART_DRE ; UART0/UDR Empty Handler
$0028 jmp UART_TXC ; UART0 TX Complete Handler
$002A jmp ADC ; ADC Conversion Complete Handler
$002C jmp EE_RDY ; EEPROM Ready Handler
$002E jmp ANA_COMP ; Analog Comparator Handler
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10. Interrupts mapping $0030 jmp TIM1_COMPC ; Timer1 CompareC Handler
$0032 jmp TIM3_CAPT ; Timer3 Capture Handler
$0034 jmp TIM3_COMPA ; Timer3 CompareA Handler
$0036 jmp TIM3_COMPB ; Timer3 CompareB Handler
$0038 jmp TIM3_COMPC ; Timer3 CompareC Handler
$003A jmp TIM3_OVF ; Timer3 Overflow Handler
$003C jmp UART1_RXC ; UART1 RX Complete Handler
$003C jmp UART1_DRE ; UART1/UDR Empty Handler
$0040 jmp UART1_TXC ; UART1 TX Complete Handler
$0042 jmp TWI ; ADC Conversion Complete Handler
$0044 jmp SPM_RDY ; Programme Memory Ready Handler
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11. Using interrupts Global enable via the status register
You must first tell the processor that it should use interrupts
Masks to work at bit level within devices
Control registers to select type of signal
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12. Global Level: Status Register
In order to use any interrupts on ATmega128 you must set the ‘I’ bit in the status register (SREG) using the command sei
By now you should know what The V,N,Z,C bits are. D7 D6 D5 D4 D3 D2 D1 D0 I T H S V N Z C
SREG
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13. At device level:
EIMSK – use to mask which external interrupts are used
EICR – used to control how external interrupts are recognised
EIFR – flags to show which have been triggered
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14. The Timer/Counter Mask Register
OCIE2:Timer/Counter2 Output Compare Interrupt Enable
TOIE2:Timer/Counter2 Overflow Interrupt Enable
TICIE1: Timer/Counter1 Input Capture Interrupt Enable
OCIEA1, OCIEA2:Timer/Counter1 Output CompareA,B Match Interrupt Enable
TOIE1: Timer/Counter1 Overflow Interrupt Enable
OCIE0: Timer/Counter0 Output Compare Interrupt Enable
TOIE0:Timer/Counter0 Overflow Interrupt Enable D7 D6 D5 D4 D3 D2 D1 D0
OCIE2
TOIE2
TICIE1
OCIE1A
OCIE1B
TOIE1
OCIE0
TOIE0
TIMSK
Note: Timer3 settings in ETIMSK
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15. Timer/Counter0 Control Register
CTC0: Clear Timer/Counter on Compare Match
COM00 / COMM01:Compare Output Mode, Bits 1 and 0
PWM0: Pulse Width Modulator Enable
The timer pre-scale factor : CS02; CS01; CS00 D7 D6 D5 D4 D3 D2 D1 D0
Res
PWM0
COM01
COM00
CTC0
CS02
CS01
CS00
TCCR0
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16. Pre-scaling the Timer via TCCR0f
Counter
TOSC1
ASO f
f/8
f/32
f/64
f/128
f/256
f/1024
CS00
CS01
CS02
Multiplexer
Clock Generator
Timer0 Clock
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17. Pre-scaling the Timer via TCCR0
CS02
CS01
CS00
Frequency (PCK0 = 8 MHz)
Timer/Counter0 is stopped 1
PCK0
PCK0/8
PCK0/32
PCK0/64
PCK0/128
PCK0/256
PCK0/1024
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18. Timer/Counter0 TCNT0
8-bit registers which contain the value of the Timer/Counters.
Both Timer/Counters are realized as up or up/down (in PWM mode) counters with read and write access.
If the Timer/Counter is written to and a clock source is selected, it continues counting in the timer clock cycle after it is preset with the written value D7 D6 D5 D4 D3 D2 D1 D0
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19. Example Program
In the example program the 8 Mhz clock of the ATmega128 is pre-scaled by 256 and the timer zero is loaded with $B2.
The counter overflows ($00) every 125 x 256 x ($FF-$B2 + 1) nsec (approx every 2.5 msec) and causes an interrupt.
Every 200 interrupts a counter is incremented and the result is displayed on the PORTB LEDs.
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20. Interrupt Jump table
At Program Reset your program jumps to the initialization
jmp Init ;2 word instruction to set correct vector
jmp xxx ;next interrupt
nop ;use this two liner if no routine available reti .
jmp TIM0_OVF ; Timer 0 Overflow interrupt Vector
nop ; Vector Addresses are 2 words apart
reti
when Timer0 overflows the interrupt controller jumps to this location
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21. Interrupt initialization;
; **** Timer0 Setup Code ****
ldi r16,$06 ; Timer 0 Setup
out TCCR0, r16 ; Timer - PRESCALE TCK0 BY 256
; (devide the 8 Mhz clock by 256)
ldi r16,$b2 ; load timer with n=178
out TCNT0,r16 ; The counter will go off
; in (256 - n)*256*125 nsec
; **** Interrupts Setup Code ****
ldi r16, $01 ; Timer Interrupt Enables
out TIMSK, r16 ; T0: Overflow
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22. Interrupt Service Routine
When an interrupt occurs : D7 of SREG is set to ‘0’ and the program jumps to the interrupt service routine
TIM0_OVF:
in R4,SREG ;save SREG
inc r17 ;increment cycle
nop
cpi r17,$C8 ;compare cycle with 200
brne again ;if <> jump to again
out PORTB, r18 ;send bits to PORTB
inc r18 ;Increment the portB number
clr r ;clear cycle and start counting
;200 interrupts
again:
ldi r16,$B2 ;reload timer value
out TCNT0,r16
out SREG,r4 ;restore sreg
reti
It is a good idea to save the information stored on the status register
and restore it at the end of the program
‘reti’ sets the interrupt bit, in the SREG, back to ‘1’ so the next interrupt can be serviced
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23. Main Program main: ;wasting time ; nop rjmp main ;loop
The timer is counting in the background and when it overflows it causes an interrupt forcing the program to execute the interrupt service routine.
After the interrupt is handled we call reti and the program comes back in the loop where it was when the interrupt occurred
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24. Interrupt Service Routines
Interrupts can occur at any time
They are asynchronous to the operation of the rest of your program
Your program may be doing something else important
The interrupt service routine should leave the program in a state so that in can continue running after the reti
General Hints
Keep the service routines short – don’t do any heavy computation, don’t do long and involved I/O if you can avoid it
Better to simply flag that the interrupt has happened so that elsewhere in your program you can deal with it
Make sure to save and restore any registers that you use – including the status register
Only if you can guarantee that you are not using a register elsewhere in your program can you avoid saving and restoring
Very unpredictable effects can happen if you don’t do this
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