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magestik
Joined: 16 Oct 2008
Posts: 59
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| input levels RS232 |
 Posted: Thu Apr 02, 2009 2:14 am |
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Hi,
Just a quick question, I am looking at the 18F4550 datasheet, i'm looking for the input high voltage for C6 and C7.
I have a signal on the receive pin @ 3V, will it be enough for the high input of the receive pin ? I am a bit lost with TTL buffers, Schmitt Trigger buffers and so.
Thank you. |
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Ttelmah
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| Posted: Thu Apr 02, 2009 2:25 am |
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Not if you are running the PIC at 5v.
Start by looking at the part called 'I/O ports'. Go to table 10.5 'PORTC summary'. Look to pn C7. Read the line for it being used as the serial RX. Says:
The '1', is the required TRIS setting. The 'ST' says what type of input buffer it has (Schmitt).
Now look at the 'electrical characteristics'. Go to line D041. Tells you what the 'high' level required is for a Schmitt input.
Best Wishes |
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magestik
Joined: 16 Oct 2008
Posts: 59
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| Posted: Thu Apr 02, 2009 2:30 am |
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| I see, there must be a pin which would tolerate 3V as high logic voltage no ? |
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RLScott
Joined: 10 Jul 2007
Posts: 465
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| Posted: Thu Apr 02, 2009 5:58 am |
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| magestik wrote: | | I see, there must be a pin which would tolerate 3V as high logic voltage no ? |
3v will probably be recognized as logic high in a 5-volt system, but it is not guaranteed. The minimum voltage to be recognized as a guaranteed logic high is 0.7 Vdd or 0.8 Vdd, depending on the type of input. Even 0.7 Vdd is 3.5 volts. If you assume that 3 volts will work, you might be lucky and have it appear to work at first. But then you may see sporadic failures as the input sometimes sees 3 volts as low. It may even turn out that 3 volts will always be seen as low. It will be very difficult to debug.
If you need to reliably see 3 volts as logic high, then you should add some additional buffer circuitry that is appropriate for your application.
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Robert Scott
Real-Time Specialties
Embedded Systems Consulting |
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