[time-nuts] Clocking a PIC16F628A from a Rubidium Standard

[Steve .]

As far as the atmel (avr):

Almost all my projects use AVR microcontrollers, Due to the R&D nature of
my work I've /always/ pushed the envelope. I use data sheets as a guideline
and nothing else. Years ago I poked around publically at the avr forum
about the idea of exploiting undocumented instructions. People in general
were not interested in anything out documentation parlance. *Burn in * is
second nature to me. Both profiling and application.

As far as powering down a MCU while keeping a clock running, The easiest
solutions i've found are to use a field effect, i prefer jfet but signal
fets would work just as well. This way you don't need to be concerned about
back feeding power from IO or in this case the clock/oscillator input.

Steve

On Fri, Nov 25, 2011 at 4:21 PM, Robert Atkinson <robert8rpi at yahoo.co.uk>wrote:

>
>
> 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 <attila at kinali.ch>
> To: Discussion of precise time and frequency measurement <
> time-nuts at 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 <kevin at 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?
>
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