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

Uwe Klein uwe at klein-habertwedt.de
Tue Sep 24 13:10:38 UTC 2013

detha 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
>>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.
A lot of 3.3V logic comes in 5V tolerant. Look up the datasheet.

For a resistive divider ( that includes the diode hacks ) you need
to compensate the input capacitance i.e. a combined resistive/capacitive divider
for good rise and fall times.

then use shottky diodes.

one variant is to use a reasonably strong pullup to the input supply voltage
and a diode ( anode * resistor * input )
pull down on diode cathode ( usually higher current capability from the driver side) for low.
High voltage on diode cathode will leave the input to go up by way of the pull up resistor.


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