Hi again ;-)
The 12F625 is a bit old, it has not as many features as the more recent
controllers. One of the drawbacks here is, that it has only a fixed internal
speed of 4MHz. Therefore you have only options to have Timer1 to run at 1000,
500, 250 or 125 kHz. So, if you need to measure times longer than 1/4s, you
will have to extend the timer in software.
But hey, give it a try for "usual" speeds between 1/30 and 1/500 and extend it
once you're there :-)
Greets,Kiste
P.S.: If a digital camera exposes 1/8000s, that is the electronic shutter
speed, surely not mechanical.
Am Samstag, 5. September 2020, 13:21:43 MESZ hat Jes
<[email protected]> Folgendes geschrieben:
Hi Oliver, you are right on everything.I got some 12F675 that if not using
capture mode, I think they can be used in Timer1 gate mode (have to check it
yet).1/8000 means 125 useconds, I put that limit because some of the modern
digital cameras have such high speeds, but in the practice for most of my
cameras 1/1000 would be more than enough, so your recommendations are fully
valid.Regarding phototransistors, you are also right. Photodiodes are faster,
but normally they need an opamp behind to drive the digital ports. I got some
BPW40 phototransistors, with a supply of 5volt a load of 100 ohm and a current
of 5mA use to have raise and fall times of less than 50 uSec, that I think is
enough for the 1/1000 case (1 mSec, 0.05% of the signal), so at first I'll try
that. If I find any problems I'd switch over to the photodiode. And for the
lighting I can use a standard led bulb DC driven, instead of the laser, you are
right, the laser can overdrive the sensor, and may not be necessary.Kind
regards and many thanks for the help.Jes.
On Sat, Sep 5, 2020 at 12:47 PM 'Oliver Seitz' via jallib
<[email protected]> wrote:
Hi Jes!
PIC12F1822 would be a nice candidate. 8 pins, one I2C, up to 32MHz on Timer1.
For your application I wouldn't use "Capture", but "Timer1 Gate".
As resolution, you could think about 64µs (or 15625 Hz), it is 6.4% at shutter
1/1000. Without crystal you won't get more garanteed than 2-3% accuracy anyway.
Without using interrupts, you could measure as long as (65536*64µs=) 4,19
seconds.
Ok, well if you have speeds down to 1/8000 (which I have never seen ;-) can
central shutters do that? ) you would need to use the interrupt method to
extend the timer.
And... phototransistors are quite slow, especially then overdriven by something
like a laser. Might be worth thinking about using a photodiode.
Greets,Kiste
Am Samstag, 5. September 2020, 12:08:40 MESZ hat Jes
<[email protected]> Folgendes geschrieben:
Thanks Rob. I'm not in a hurry, just learning and trying to get profit from
what I learn.My project is a simple shutter speed meter for old film cameras
(which I collect and repair).There is a little led laser pointing to a
phototransistor sitting on the film plane and going through the lens socket.
Max shutter "speeds" (the term is used in photography) are of 1/8000 of sec. On
the other end, time can be a long exposure of minutes or even hours, but
normally measuring up to 5 sec should be enough to certify the accuracy of the
shutter. Most pictures are taken in a range between 1/30 sec and 1/500 sec.
Many old cameras have a restricted range, not beyond 1/250 sec.I would like to
have an autonomous unit (not tied to a, computer) so I would need only a few
pins, just one for the signal and two for an small I2C display, like the ones
powered by the SSD1306 :O)I was following the thread because that.Now I have to
find out if one of the small 12F units is able to do that. Would be a perfect
fit for my needs.Kind regards.Jes.
El sáb., 5 sept. 2020 10:17, Rob CJ <[email protected]> escribió:
Hi Jes,
Since my previous project was finished I was thinking of prototyping this
frequency counter in JAL. My idea is to use it in combination with an SSD1306
display and test it using the library I created for the Si5351 although the
range will be limited of this frequency counter.
I think of making it 'auto ranging' as to increase the frequency range indeed
by changing prescaler values as you mentioned to measure lower frequencies. I
do not yet know how low it can measure.
I might even post it on Instructables as prototype. I saw some examples there
based on Arduino but most of them are very simple, that is no auto-ranging and
so limited in the range it can handle.
I started today, not sure how long it wil take but when it is done I will post
the source file on Jallib or on Instructables.
BTW I will base it on a PIC16F1823 since that is the one I often use in my
projects but you can use other PICs as long as it has the capture function. As
said I do not know how long it will take so don't wait for it🙂.
Kind regards,
Rob Van: [email protected] <[email protected]> namens Jesito's
μFabLab <[email protected]>
Verzonden: zaterdag 5 september 2020 09:55
Aan: jallib <[email protected]>
Onderwerp: Re: [jallib] Some help needed to measure pulse length Many thanks
Rob. I have to figure out how to do it. Is the timer working at the MCU clock
speed?. If so the 16 bit value might be too small to hold the pulse length for
i.e. a 5 sec pulse, isn't it?. I suppose some prescaling should be done
there... Have to investigate and read more on that.Kind regards.Jes.
El jueves, 3 de septiembre de 2020 a las 18:51:51 UTC+2, [email protected]
escribió:
Hi Jes,
I just checked but I was mistaken, there is a procedure that calculates a value
for the timer based on a frequency.
If you want to measure the length of a pulse you can use the capture mode of a
timer which is available in many PICs, for example the 16F1823.From the
datasheet:Capture mode makes use of the 16-bit Timer1resource. When an event
occurs on the CCP1 pin, the16-bit CCPR1H:CCPR1L register pair captures
andstores the 16-bit value of the TMR1H:TMR1L registerpair, respectively. An
event is defined as one of thefollowing and is configured by the CCP1M<3:0>
bits ofthe CCP1CON register:• Every falling edge• Every rising edge• Every 4th
rising edge• Every 16th rising edgeWhen a capture is made, the Interrupt
Request Flag bitCCP1IF of the PIR1 register is set. The interrupt flagmust be
cleared in software. If another capture occursbefore the value in the CCPR1H,
CCPR1L register pairis read, the old captured value is overwritten by the
newcaptured value.
Kind regards,
Rob
Van:[email protected] <[email protected]> namens Jes
<[email protected]>
Verzonden: donderdag 3 september 2020 13:45
Aan: [email protected] <[email protected]>
Onderwerp: Re: [jallib] Some help needed to measure pulse length Thanks Rob, I
look for it, just what I needed to know :0)
El jue., 3 sept. 2020 12:01, Rob CJ <[email protected]> escribió:
Hi Jes,I thought there is already a Jal library to measure the width of a pulse
created by Matt. Would that do the job?
Met vriendelijke groet,Rob Jansen From:[email protected]
<[email protected]> on behalf of Jesito's μFabLab <[email protected]>
Sent: Thursday, September 3, 2020 10:49:48 AM
To: jallib <[email protected]>
Subject: [jallib] Some help needed to measure pulse length Hello,I'm new to JAL
and I would like to measure the length of some pulses that can be as long as 5
sec and as short as 1/8000 of second.I have a handful of different pics, but
ideally I would like to use a small one, I just need one pin for the pulse, two
more to interface to an I2C small display and one more to reset and restart the
measure.Could anyone point me to an example that could guide me through this?.I
love the cleanless and simplicity of JAL.Thanks in advance.Jes.
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