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Why infrared receiver modules have a normally high output

 
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NEC1



Joined: 03 Jul 2010
Posts: 56

PostPosted: Sat Nov 10, 2012 2:32 am    Post subject: Why infrared receiver modules have a normally high output Reply with quote

I found the reason why the vast majority of infrared receiver modules have a normally high output when no signal and/or carrier is detected is because the output of the receiver is usually connected to an interrupt line (in many cases, active low) on a microcontroller.

So when constructing an interface, the output of the infrared detector should be normally high on no signal/carrier and the software should be programmed as such.
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cauer29



Joined: 03 Feb 2010
Posts: 235

PostPosted: Sat Nov 10, 2012 10:29 am    Post subject: Re: Why infrared receiver modules have a normally high outpu Reply with quote

NEC1 wrote:
I found the reason why the vast majority of infrared receiver modules have a normally high output when no signal and/or carrier is detected is because the output of the receiver is usually connected to an interrupt line (in many cases, active low) on a microcontroller.

So when constructing an interface, the output of the infrared detector should be normally high on no signal/carrier and the software should be programmed as such.


Since just about every microcontroller worth its salt for the last 25 years allows interrupt on high or low, that's not it. More likely it has to do with low power operation. An open collector active low output doesn't draw any power when there's no IR present.

A.A.
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Kevin Timmerman
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Joined: 09 Jan 2007
Posts: 142
Location: West Michigan

PostPosted: Sat Nov 10, 2012 5:25 pm    Post subject: Reply with quote

The IR receiver module would typically be connected to a timer/counter unit input. That allows the exact time of a rising and/or falling edge to be captured and an interrupt triggered conditionally. For some protocols such as Sony and RC5/6 it is necessary to know the time of both edges, for NEC and similar it is possible to decode with only falling edge times, but having both can make the decoder more robust.

When these IR modules first became available the current chip technology typically used all N channel transistors. TTL, NMOS and many common analog chips from the 70's for example. This was much easier to manufacture and thus had lower cost than chips using both N and P channel transistors (CMOS for example). So if you build a chip with just N channel transistors it is common to have open collector outputs or outputs that can sink more current than they can source. This is why you (still) see at lot of busses that are actively driven low and passively pulled high. To improve fan-out and reduce power dissipation in these systems the inactive state is high and the active state is low - the logic levels are inverted.

The active low open collector output also allows for the IR module to operate at a different voltage than the microcontroller.

Executive summary: Lower cost, lower power, easy interface to almost any logic level, legacy
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