How do I wait for keypress without loop?


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Hello. I've written some C++ to control the keys for my 635, but I do not know how to wait for a keypress without being in a loop. It works except it takes up 90+% of the CPU.

Hopefully someone can show me how to get rid of this problem! Thanks!
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In general, there are two (or three) ways to wait for an event:

1. polling in a loop

2. respond to an interrupt

(3. wake up from sleep)

Why is polling a problem? If your cpu has nothing else to do while waiting for a keypress, then 90%+ cpu usage is not an issue. If power consumption is important, then use a cpu wait/stop/etc mode, and interrupt to wake up.

If you supply more info, a better answer might be given.

CF Tech

Try putting a Sleep(50) in your loop. Thit will allow you to poll at about 20Hz. The Sleep(50) gives control back to the OS, so the CPU usage shoulf go down.


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I have the same problem and I have tried sleep(1) in my loop.
With that configuration I poll every second if a key has been pressed.

If yes, I switch to usleep(100000) and then I poll every 100ms until the user has exit my menu. Then i go back to sleep(1).
That all works fine but I get problems when I listen for keys, fanspeeds and temperature at the same time.

When all of that stuff is enabled and I poll with sleep(1), temp and fans are doing pretty well but it takes sometimes up to 5 seconds or multiple press of the same key until a pressed key is recognized and that's far to much!

Why is fan/temp reporting slowing down the system that much? Are there any solutions or what am I doing wrong?

Originally posted by Guinness I have the same problem and I have tried sleep(1) in my loop.
With that configuration I poll every second if a key has been pressed.
Seems to me that would result in a one millisecond sleep (1000 polls per second), not one second.


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I am using C and Fedora Core 5, int the product we will use FC4.

If I use sleep(1) it is a second and sleep(50) --> 50 seconds. That's why I have to use usleep() to get a timespan smaller than 1 second.
Maybe that's because I use Linux?!

The LCD will be built in a decoder, because of that a cpu load of more than 1-3% for polling the keys is unacceptable.
Interrupting the decoder is also a bad idea because it is decoding a video stream in realtime...that all makes it so difficult!
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CF Tech

My understanding is that "Sleep()" is Windows, in mS, and "sleep()" is Linux / unix in S.

Can you hook the RS-232 incoming character interrupt? If so, just run "check_for_packet()" in that ISR, or have that ISR somehow trigger a "foreground" call to "check_for_packet()" to look for the keys.

If a key packet is 5 bytes, it would be no big deal to call check_for_packet() 5 times (once for every incoming byte), on the last call it will return the packet. This is nearly how the module's firmware does it.