[time-nuts] Oncore question

[SAIDJACK at aol.com]

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.