I have read that the postscalar of a timer specifies how many times the counter has to overflow inorder to get an interrupt.
But i have a doubt there.
So what i understand is if i put 0x55 and start timer with postscalar as 2, then timer will count from 0x55 to 0xFF and then again 0x55 to 0xFF and generate an interrupt.
Consider a case that i start the timer in an external inetrrupt. My requyirement may be to get the timegap between two interrupt. I start the timer in first interrupt, then read the timer in the next interrupt.
but if i have put postscalar then i will get the wrong time right.
I just used this as an example to make my question clear.
Edit: So will there be any issue if a timer value is read when postscalar turned ON
Usage Context: To get time difference between two interrupts
No. PostScale - Pre-Scale divide the clock input/output so you can sample at lower frequencies or intervals, depending on the application where you need more count than available. Let's say you have a XTAl of 8MHz with a Pre-Scaler of 1:8 (found on many PICS), you won't sample at 8MHz but at 1MHz.
Addind a pre-scaler - Post-Scaler will change the time between your 2 interrupts, surely. But that won't affect the reading of the counter value, assuming you count a variable each time there's one of the 2 interrupts on. You will simply count slower, or faster, depending on which timer you are using (most of them only have a pre-scaler option).
Related
I want to solve a very particular problem with an Arduino, running on a ATmega328
I have timer1 run in fast PWM mode for generating a PWM Signal with given Period an duty cycle. The duty cycle is set by OCR1A: So at timer restart the level of the output pin gets high and after the duty period it gets low. This works.
Additionally I want to carry out an analog measurement exactly some time after the rising signal. So I enabled the OCR1B interrupt and define the time by writing to OCR1B. When the timer reaches the value in OCR1B the interrupt handler is invoked and the measurement done. This works.
Now I want to do this ADC conversion twice at different counter times, say my_OCRB1 and my_OCRB2. But there is only on OCR1B Register I can use. Is it ok to prepare the content of OCR1B to the next value my_OCRB2 after the first ADC conversion at is finished? Will it work and again rise an interrupt, when the timer is still counting up?
Is there a better solution?
I have a rotary encoder with STM32F4 and configured TIM4 in "Encoder Mode TI1 and TI2". I want to have an interrupt every time the value of timer is incremented or decremented.
The counting works but I only can configure an interrupt on every update event, not every changes in TIM4->cnt. How can I do this?
In other words: My MCU+Encoder in quadrature mode could count from 0 to 99 in one revolution. I want to have 100 interrupts in the revolution but if I set TIM4->PSC=0 and TIM4->ARR=1, results 50 UPDATE_EVENTs, so I should set ARR=0 but it does not work. How can I sole that?
To get 100 interrupts per revolution keep PSC=0, ARR=1, setup the two timer channels in output compare mode with compare values 0 and 1 and interrupts on both channels.
Or even use ARR=3 and setup all four channels, with compare values of 0,1,2 and 3. This will allow to detect the direction.
Normally, the whole point of using the quadrature encoder mode is counting the pulses while avoiding interrupts. You can simply poll the counter register periodically to determine speed and position.
Getting interrupts on every encoder pulse is extremely inefficient, especially with high resolution encoders. Yours seems to be a low resolution one. If you still think you need them for some reason, you can connect A & B into external interrupts and implement the counting logic manually. In this case, you don't need quadrature encoder mode.
My processor is an STM32F437ZGT6 and I wish to count two different pulse trains (RPM). The range is quite wide, I may have an engine that idles at 150 rpm and we get a pulse from the cam, so 0.5 pulses per revolution, or 1.25 pulses per second. At the other extreme I may need to count 460 flywheel teeth at 3000 rpm, 23000 pulses per second. I have a prescaler available so I can divide the external event by up to 8 but even so this become too intense at higher speeds because every event or eight event causes an interrupt.
One alternative I am considering would be to have one timer use the external event as the clock and it would just count events within a time window. My difficulty comes from determining how to use another timer to control the window by setting and clearing CEN or some similar action.
In RM0090, section 18.3.15 Timer synchronization the example shows one timer controlling another, timer 1 controlling timer two. I thought that may be useable but althought I did not read otherwise I don't see that any two timers could be paired. The signal I am interested in actually feeds two timers. TIM1 ch1 and TIM9 ch1.
Any suggestions would be appreciated as I don't want to cobble up some Rube Goldberg scheme where one timer fires off an ISR and then the ISR opens and closes the time window.
I should have noted that a lookup table is provided that provides the expected engine speed and the number of pulses per revolution.
Thanks,
jh
If you want to just count external events, you can select external clock source for timer. (Point "Clock selection" of reference manual). SPL should have an example.
And read count from Tim CNT register every time you need.
The problem here is to read counts often enough.
Usually auto-reload register is 2 bytes so you have up to 2 ^ 16 counts before overflow, and loosing counted value.
Timers 2 and 5 have 4 bytes auto-reload registers so you have up to 2 ^ 32 counts.
If you need more then 2 ^ 32 counts you have at least two ways:
- Timer cascade, by setting one timer event as a clock for another.
You can find this in the reference manual as "Using one timer as prescaler for another timer".
Cascading offers you up to 2 ^ 64 timer.
There is an example for SPL in "TIM_CascadeSynchro" folder.
- Less beautiful, but more easy way is to create a counter variable and, increment it in timer irq handler.
Number of counts may be found as cnt_variable * TIMx-> ARR.
Several cascaded variables give the unlimited counter).
Thanks for the post. I will try to add a little detail. RPM 1 is fed into TIM3 ch2 and TIM4 ch1. RPM 2 is fed into TIM1 ch1 and Tim9 ch1. Both have a range of 1.25 pulses per second up to 30000 pulses per second. I am given the number of pulses per revolution which can range from 0.5 to 460 and the expected engine rpm, 150 - 3000 rpm so I can scale things a bit. The reason for feeding two different timers is to be able to use different counting techniques based on speed (pulses per second). For low speed I can capture events (pulses) and grab the timer count using an ISR. But when the pulse count gets high I want to use a different method so as not to incur more than 1000 interrupts per second per channel. So my idea there is to have one timer control another. One timer would simply count events without generating interrupts. The second counter would control the period of time that the first timer would be allowed to collect events.
Thanks,
jh
Seems like you need: timer synchronization with enabling/disabling slave timer according to the trigger output of master timer.
Description can be found in the following sections of RM0090:
18.3.14 Timers and external trigger synchronization in paragraph Slave mode: Gated mode
18.3.15 Timer synchronization in paragraph Using one timer to enable another timer
Also good explanation can be found in TIMx register section for registers TIMx_SMCR: bits TS and SMS; TIMx_CR2: bits MMS.
TIMx internal trigger connection (tables 93, 97 and 100) сontains possible connections of the trigger output of one timer with the input of another. Timers that you can use as master are marked in the picture below:
TIM_ExtTriggerSynchro example from SPL library can be used for code copy-paste.
I think the best way is:
Set RPM pins as external clock source for slave timer.
Set enabling/disabling of slave timer from the output compare of master timer. So changing TIMx_CCRx register value you can change duration of measurement.
Set master timer interrupts on update event (maybe on few events TIMx_RCR register).
Do all the calculations in master timer interrupt handler
Also it seems to me that you can just use 16 bit timer as RPM counter. Even on 30000 pulses you will have overflow every 2^16/30000 = 2,18 seconds which is rarely rare for STM32F4 clock frequencies. And use other timer, with, for example, 2 second period, interrupt for calculations.
Good luck!
I am implementing a time counter on my atmega 328p. I looked on the implementation of arduino millis function and I am bit confused, why they use Timer Overflow Interrupt which is executed every 1.024 ms (freg = 16MHz, 64 prescaling), when they could use Output Compare Match Interrupt which can be set up to trigger exactly every 1ms (OCR0A = 249). Is there any advantage to use Timer Overflow Interupt and do some corrections to counted ms over Output Compare Match interupt that is executed exactly every 1ms? Or why they are using it?
The counter value TCNT is used for calculation of microseconds beyond interrupt. Using compare match for defining TOP value would generate exact interrupt but complicates finer micros calculation as TCNT is reset. Using compare match for non TOP value (for PWM generation) does not generate periodic 1ms interrupt.
I'm personally using as second timer for sampling TOP value defined by OCRxA register.
Im using Ethernut 2.1 B and I need a C program that outputs a clock signal at the timer 1 output B, with other words on output OCIB. The frequency of the clock signal should be at 1.0 kHz.
Anyone know how this could be done?
You need to look in COM bits for your timer. For instance, for Timer0 (8-bit), the COM bits are set in the TCCR0 register. Probably the setting you'd be interested in is
TCCR0 |= (0<<COM1)|1<<COM0); // Toggle OC0 on compare match
This will toggle the OC0 (pin14) line when timer reaches the specified value.
Which timer you use depends on the precision you need: obviosely the 16-bit timers can give you more precise time resolution then the 8-bit timers.
The setting of the registers for your specific frequency (1Khz) depends on the clock speed of your chip, and which timer you are using: the timers use a pre-scaled general clock signal (see table 56 of the datasheet for possible values). This means that the prescaler settings will depend on your clock speed, and how high you want to count. For most precision you will want to count as high as possible, which means the lowest possible prescaler setting compatible with your timer's maximum value.
As far as where to start, generally, reading the datasheet is a good place, but googling "AVR timer" can also be very helpful.
It seems to be based on the Atmel ATmega 128, so read that CPU's data sheet to figure out how to program the timer hardware.
Not sure if this microcontroller supports directly driving an output from a timer, if it doesn't you're going to have to do it in software from the interrupt service routine.