gnupic: Re: [gnupic] Line recievers
Subject:
Re: [gnupic] Line recievers
From:
Robert Pearce ####@####.####
Date:
25 Oct 2006 20:48:43 +0100
Message-Id: <kGtH$mDD$7PFFwDl@daniel.huneausware.local>
On Wed, 25 Oct 2006, Dan Andersson ####@####.#### wrote :
>
>Why even use a chip?
>
>Use pin diode(s) instead.
Why bother with the diode?
All the PICs I've used (admittedly none of them 18F series) have
internal clamp diodes to both supply rails. The Microchip application
notes recommend connecting higher voltage inputs with a simple series
resistor. Because the PIC inputs are very high impedance you can use a
fairly high value resistor (220K).
That said, as I understand it the application uses a mechanical
micro-switch to +24V, otherwise open circuit. For this you want to
provide a decent wetting current, and you _need_ to provide a pull-down
for the PIC input. I would solve both problems in one step with a second
resistor, from the switch to ground. This should be a smaller value,
probably around 2K2 (giving a 2mA switch current).
+24V |
|
o |
|| Sw
o | |
| |
|-----\/\/\/\/-----| PIC
| 220K |
/ |
\
/ 1K2
\
|
GND |
--
Rob Pearce http://www.bdt-home.demon.co.uk
The contents of this | Windows NT crashed.
message are purely | I am the Blue Screen of Death.
my opinion. Don't | No one hears your screams.
believe a word. |
