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

David Taylor david-taylor at blueyonder.co.uk.invalid
Mon Sep 16 06:26:06 UTC 2013

> 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
> enough.


(a) the diode would prevent the source from sinking current.  Sometimes 
the inputs to digital circuits will have a pull-up resistor to +Vcc 
(i.e. +5 or +3.3 V), and the diode would prevent the input being pulled 
down to zero.

(b) having a suitably high resistance in circuit will limit the current, 
and may help prevent damage if the voltage levels weren't quite as expected.

As David Lord helpfully pointed out, the Sure board also has the signals 
you need at 3.3 V level, so a direct connection should be possible.

>> 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.

The Adafruit unit also offers NMEA sentences over its serial line.  With 
the Raspberry Pi, there are only two USB ports, and many GPIO pins, so 
you may prefer to use a serial rather than a USB feed for the NMEA data, 
but both do work (tested here using gpsd).

Please bear in mind that I'm not suggesting that a Raspberry Pi and 
Adafruit GPS is the ultimate NTP machine - far from it - but it may well 
be a cost-effective, low-power add-on to an existing network, 
particularly for the OP who requires to sync Windows PCs.  Having a 
local stratum-1 server (or more) is a significant improvement over 
Internet sync alone.
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