[ntp:questions] Trying to use Dimension 4 time keeper

unruh unruh at invalid.ca
Mon Sep 16 06:27:22 UTC 2013

On 2013-09-16, detha <detha at foad.co.za> wrote:
> On Sun, 15 Sep 2013 18:56:26 +0100, David Taylor wrote:
>> On 15/09/2013 18:37, unruh wrote:
>> []
>>> Yes, I saw it. Not sure what the voltages are on the Sure unit -- the
>>> direct output of the PPS but I think 5V which is still a bit high. I
>>> guess one could put a diode in series with the line to bring it down to
>>> 3.3V.
>> I would recommend using a restive divider - perhaps 10K and 4.7K to start
>> with - and monitoring the voltage into the Raspberry Pi.  Using resistors
>> provides a current limit, and allows the voltage to get down to zero.  If
>> the voltage is well below 3.3V, the upper 4.7 K resistor could be reduced.
>>  Best to ask on one of the Raspberry Pi support groups as someone has
>> likely already done the measurements.
> http://jamesreubenknowles.com/level-shifting-stragety-experments-1741
> might be of interest. For converting serial signals it works fine,
> for PPS where one worries about being 100ns off maybe not so much.
His input impedence has a capacitor in it (about 100pf it looks like).

            /|          Z
                        Z  Input impedence

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