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Old 12-26-2012, 08:47 PM   #16
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I just figured it out today, see this thread.
Good works Kevin, thank you!
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Old 12-26-2012, 08:54 PM   #17
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You could always wear sunglasses while reading your Glo in the dark...
Yes, that's a way, but wearing glasses is some uncomfortable when I'm in bed.
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Old 12-26-2012, 09:02 PM   #18
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Sheeshh,....

Their most likely method of controlling the LED brightness is using PWM. There will be no change in the flashing rate when using PWM as the brightness change will be done by modifying the amount of time the LED is on during each cycle.

If they are using PWM at 1KHz (a 1000 microsecond cycle width), I somehow doubt that anyone is going to notice any strobing effect or suffer any ill effects. Now if they were using between 5 and 30 pulses per second, you might have a case (and for the photosensitive epileptic, a seizure).
Hi David, according to Kevin's description in this thread,http://www.mobileread.com/forums/sho...d.php?t=200706
,
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And while there is one light level lower, level 1, that the Glo never uses, setting the frontlight to that level causes flickering that is very noticeable in a dark room
That is to say, though the flashing frequency is not changed, but the feel to eyes will be flickering because of shorter LED-on time in one cycle?

Last edited by xuyn2003; 12-26-2012 at 09:08 PM.
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Old 12-26-2012, 11:34 PM   #19
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You could always wear sunglasses while reading your Glo in the dark...
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Out of curiosity, why would you think that using PWM and going too low might make 'strobing' apparent? Most of the PWM implementations I am aware of use a constant cycle time (cycle = on time + off time).

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I've just always noticed some strobing / 'unsteadyness' in LED backlights on very low settings on other devices. Just assumed that's from the pwm interval being too long.
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Old 12-26-2012, 11:57 PM   #20
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Strobing is usually a sign that the PWM frequency is way too low, or that they should have used a better cap to filter or smooth the output voltage -or, worse yet, that software PWM is failing due to variable response time in a system which does not employ proper multi-tasking. There should be no requirement for high speed changes in a simple brightness circuit, so smoothing would be desirable. Further, most micro controllers available today provide for one or more timer/counters capable of hardware PWM which is greatly preferred.
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Old 12-27-2012, 08:37 AM   #21
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Originally Posted by TechniSol View Post
-or, worse yet, that software PWM is failing due to variable response time in a system which does not employ proper multi-tasking.
I'm getting the feeling that the Glo uses software PWM. I've noticed that when turning pages (or doing anything that requires serious CPU cycles), the backlight blinks (sort of). It's a very fast blink, but it's there.
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Old 12-27-2012, 09:54 PM   #22
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Could be. Hardware based PWM should be rock solid, and if properly filtered or smoothed with an RC filter should appear to gradually change even when levels are adjusted because the write to a hardware register for the PWM should be very short.
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Old 12-27-2012, 10:07 PM   #23
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I'm getting the feeling that the Glo uses software PWM.
If true, this should ruin battery consumption, because the CPU would have to be continuously running.

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Originally Posted by TechniSol View Post
properly filtered or smoothed with an RC filter
Filtering is not nearly as easy as you make it sound. Firstly, LEDs are current-controlled not voltage-controlled, so you want to smoothen current, not voltage. This immediately points to an inductor rather than capacitor, and inductors are big and expensive. Secondly, you don't want to dump the unwanted energy (the high frequency components) to a resistor, it's an immediate waste of precious battery power (as soon as you include any resistor anywhere, it might very well defeat the purpose of PWM).

I'm not saying it's all not solvable (with some compromise between cost and power waste) but in the end, I don't think I've ever seen a PWM/switchmode LED controller that would have any filtering. They usually just use frequency that's high enough instead...

Quote:
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ioctl(light, 241, brightness);
Curious about the 241 magic number. Can it be that another number controls the frequency?
I'll have to figure out if I can cross-compile on Windows too...

[edit later] BTW, I don't see any flickering at light level 1 - maybe it's too bright outside to see it... it's just like level 2 (GUI minimum) only a little bit darker.

Last edited by sysKin; 12-27-2012 at 11:23 PM.
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Old 12-28-2012, 01:19 PM   #24
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If Kobo devs are reading this, a white-on-black 'night mode', as seen on basically every other modern device with an illuminated screen, would allay most of the concern. Pics are of the new Cybook with essentially the same hardware as the Glo.
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Last edited by stewacide; 12-28-2012 at 01:33 PM.
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Old 12-28-2012, 02:28 PM   #25
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If Kobo devs are reading this, a white-on-black 'night mode', as seen on basically every other modern device with an illuminated screen, would allay most of the concern. Pics are of the new Cybook with essentially the same hardware as the Glo.
The Glo already has support for inverting the screen, it just has to be specified when updating the e-ink display. I've attached a picture of my Glo after running the e-ink code with invert enabled. But the screen doesn't stay inverted; as soon as you change the page the display goes back to normal.
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Old 12-28-2012, 07:46 PM   #26
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If Kobo devs are reading this, a white-on-black 'night mode', as seen on basically every other modern device with an illuminated screen, would allay most of the concern.
Most or some, but certainly not all - there are plenty of us out there who can't read white-on-black comfortably.
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Old 12-29-2012, 03:24 PM   #27
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Most or some, but certainly not all - there are plenty of us out there who can't read white-on-black comfortably.
Thank you for pointing that out, meera--I can't even get through a page with it set to white on black. The text swims and I have the beginnings of a headache almost instantly. Maybe it's my astigmatism causing it, I'm not sure, but something does.

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Old 12-29-2012, 06:52 PM   #28
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SysKIN,

My guess would be that they would include a current limiting resistor, or resistors, in series with the LEDs to specify maximum current allowed so the LEDs would not burn out if PWM accidentally got locked at 100% on time. If so, then varying the voltage would in effect vary the current through the circuit as I = (E - LEDvdrop) / R... I would guess a buffering cap couldn't hurt. I shouldn't have called it an RC filter as it really isn't since the LED would be in series with the resistor. At least that would be my take on it. You have to allow for situations where the LEDs might be burnt out and engineer for it in a consumer device, or the first time a processor locks up with the PWM full on it's bye-bye LEDs.
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Old 12-29-2012, 09:38 PM   #29
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I would guess a buffering cap couldn't hurt.
Ah, a cap immediately at PWM pin before the resistor? I see what you mean and maybe, but (not saying you're wrong) let me explain how this is still not easy:
If PWM is to do its power-saving job properly, in its "on" state it should have near-zero resistance. So, each pulse would immediately charge the capacitor to full voltage.
Now, if the capacitor is able to sustain the LEDs until the next pulse, the pulse WIDTH is no longer modulating anything - the LEDs are lit, from a periodically discharging capacitor, with nearly the same current pattern, regardless of pulse width.
Alternatively if the cap is only sustaining LEDs for a short fraction of the pulse period, it's hardly filtering anything. The LEDs go dark a little bit later and smoother (not a sudden switch-off), but this doesn't change the fact they're still switching between full bright and full dark at the same frequency as before.

So not only there is no smoothing in peak-to-peak sense, but there are new disadvantages: brightness control got harder because even an infinitely short pulse, as long as it charges the capacitor fully, produces some non-zero brightness, purely from capacitor's discharge.
Second problem is that the capacitor tries to charge very fast which produces very high currents from PWM pin. Very high currents multiplied by hopefully-very-small resistance of PWM pin -- that becomes a sizeable power waste which was not there before.

Disclaimer: while my education is in electronics engineering, I'm actually a software engineer so my modelling above can be detached from reality.

Hopefully my explanation makes some sense >_<

Last edited by sysKin; 12-29-2012 at 09:56 PM.
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Old 12-29-2012, 11:57 PM   #30
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sysKin,

The biggest problems I see with your reasoning is the idea that the cap can charge instantaneously -it can't, or how you'd get enough current to power 4 or 5 LEDs in parallel from one pin when most micros only source a few ma, or sink up to 20ma...

I would expect the PWM output would drive a MOSFET likely through a resistor to the gate. The MOSFET, maybe a 2N7000(200-300ma depending on manufacturer last time I used one), would probably go directly to the rail. The LED gets driven through a resistor that sets maximum current in case the FET is left on. Positioning the cap on the gate side would probably be best. Tiny cap by the way, just to smooth out spikes on either end of the PWM cycle waveform -maybe not required, but a little buffering never hurts unless you need really fast response time.

Pulse width modulation can be power saving, especially in high current applications or when supply voltage is much higher than the device's operating voltage because it's more power efficient than throwing away power on resistors, but in general it's also the easiest way to provide multiple output power levels or speeds from one microcontroller pin. It sounds great to leave the resistors out, but when the micro goes splat then the LEDs get burnt out if your PWM output gets locked on and your LEDs get excessive current because the MOSFET is continuously passing full rail voltage to your LEDs. Could be kind of embarrassing. Not sure it's worth a little power savings. OTOH, if the supply voltage is very near the LED drop, you might get away with it... maybe... for a while.
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