[time-nuts] Oncore question

Arnold Tibus Arnold.Tibus at gmx.de
Sun Nov 23 21:02:27 UTC 2008


Hi Said,
thank you very much for the additional clarification, very interesting 
behaviour. I think this does confirm roughly my assumption. When only pwr 
fails for a few hours (intermittant) have to be taken into account, super caps 
may be good for safe work during several hours up to perhaps 24 hours, 
but not beyond, is that right? Do you know at what voltage the clock does resign? 
Perhaps this could be improved with a software solving the recovery problem 
when repowering the RX. 
You describe that using LiIon batteries are not yet the ideal way, but why 
cannot higher capacities come into play (space should be not the problem 
for on ground use in fix stations)?

regards,
Arnold






On Sat, 22 Nov 2008 19:26:40 EST, SAIDJACK at aol.com wrote:

>Hello Arnold,
> 
>the current drawn by the Motorola GPS from the backup bat is not linear  
>versus voltage.
> 
>On an M12+, the real time clock takes most of the current, and will stop  
>operating after about a day or less on a Supercap. The SRAM memory keeps  
>maintaining it's contents for much longer than that.
> 
>After the RTC crystal stops, the power consumption goes down to extremely  
>low discharge rates.
> 
>I have seen our FireFox units (M12+ with supercap) still maintain position  
>and almanac over more than 1.5 weeks! Down to 0.5V or so. The Date is off by  
>that amount too of course since the RTC just stops ticking.
> 
>This presents a serious problem due to a fault in the Motorola firmware (in  
>my opinion):
> 
>The M12+ sees a "valid" Almanac, but the time and date are off due to the  
>fact that the RTC crystal just stopped a some point.
> 
>The M12+ thinks it's a different time/date than it really is, and  
>desperately tries to search the sky for a sat constellation that is not  there. It 
>doesn't even reset its internal RTC when it sees a couple of  Sats.
> 
>I have seen instances where it took a day or more for the firmware to catch  
>on, and reset the time in the RTC.
> 
>When using a small coin Lithium battery, the backup time is only 6 months  to 
>1 year or so typically, also not a very good performance.
> 
>Not a good situation, they could have solved that more elegantly for  sure.
> 
>bye,
>Said
> 
> 
> 
> 
>In a message dated 11/22/2008 16:06:57 Pacific Standard Time,  
>Arnold.Tibus at gmx.de writes:

>After  your question I did want to know myself for what time these caps would 
> serve.
>My result:
>For a backup current of 25 µA and allowing a voltage  drop during the backup 
>of 0.5 V, the data will be safe for around 5 to 6  hours,

>With only 5 µA backup current the time will be 5 times higher  (abt. 27 h) 
>not counting for leakage or other discharge factors.  


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