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

unruh unruh at invalid.ca
Sun Sep 15 19:22:05 UTC 2013

On 2013-09-15, David Taylor <david-taylor at blueyonder.co.uk.invalid> 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.

Not sure why you would want a resistive divider. The diode would do it
just as well and you would not have to worry about the input impedence
of the GPIO. The GPIO already have an input impedence and you should not
have to worry about current limits as long as the voltage was low

> But the Sure unit is so big, physically, that I would still prefer to 
> use the Adafruit instead.

It does have a usb for both power and putting out the nmea sentences.
But it certainly is bigger. 

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