Hi,
Microchip cerainly condone using input protection diodes of PIC devices as clamps. There are application notes for zero-crossing detection which connect the input to the 115V AC line via a resistor. Note that these are intentional protection diodes, not unavoidable parasitic junctions. Typical Absolute max clamp current (inc. 16F628) is +-20mA. As a side note, when using these diodes for ESD protection, Microchip recommend using 0.01uF supply decoupling capacitors close to the chip rather than 0.1uF. This reduces the peak current. Trace inductance limits the effect of more distant capacitive loading. Robert G8RPI. ________________________________ From: Attila Kinali <att...@kinali.ch> To: Discussion of precise time and frequency measurement <time-nuts@febo.com> Sent: Friday, 25 November 2011, 8:17 Subject: Re: [time-nuts] Clocking a PIC16F628A from a Rubidium Standard On Thu, 24 Nov 2011 12:05:13 -0700 Kevin Rosenberg <ke...@rosenberg.net> wrote: > Since frequency reference sine wave can exceed Vdd, you want to current > limit the external clock. For example, an unterminated TBolt puts out 0-7V > Pk-Pk. Atmel, in an app note where they hook up the pins of an AVR > to 220V mains, states the over/under voltage protection diodes should > not carry more than 1 milliamp of current. But, you should both read > the datasheet for the output voltage of the Efratom and measure Pk-Pk > voltage output at the point of your PIC. Using the protection diodes as part of the circuit is bad design practice. Use instead explicit shottky diodes (like BAT54S) for clamping. Better would be to use a resisitive divider (probably with a capacitive divider in parallel), a coupling capacitor to connect it to the clock input. You can limit the swing of the signal to less than 1V as the clock input doesnt require a big signal (when using a crystal, the "signal" can be as low as a few mV, depending on the chip) Attila Kinali -- Why does it take years to find the answers to the questions one should have asked long ago? _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.