[time-nuts] Yet Another GPSDO design - Timing on the move

Attila Kinali attila at kinali.ch
Mon Jun 26 18:09:35 EDT 2017

On Mon, 26 Jun 2017 10:43:24 -0700
"Tom Van Baak" <tvb at LeapSecond.com> wrote:

> > down. U21 is a 128KB SRAM chip for scratch space, U13 is a FeRAM chip to
> > store EFC settings (as EEPROM would wear out too fast with regular writes,
> > and I cannot guarantee having enough energy after detecting a brownout to
> > only write to EEPROM in such conditions). The other systems for the boat
> As much as I'm a fan of core memory and FeRAM, I don't understand why you 
> plan to update the EEPROM so often. How about just once a day? Or once any 
> time the DAC changes by more than 10 or 100 units. This value is informed by 
> the re-trace spec of your OCXO.

If you are worried about the number of write cycles, that's not an issue
with FeRAM/FRAM. Their spec'ed number of read/write cycles is in the
order of 1e10 to 1e14. I.e. reading/writing the same cell every second
would result in a lifetime somewhere between 300 and 3 million years. 
I am not sure I believe the latter number, but I wouldn't worry
about wear-out of FeRAM/FRAM cells under normal conditions.

The write cylces, speed, and power consumption of FeRAM/FRAM is so good
that TI has been using it as SRAM replacement for the MSP430FR family
of lowest power uC (think of years-of-runtine-from-a-tiny-coin-cell kind
of low power).

So, if continuously writing the EFC value to FeRAM simplifies the software
flow, then this isn't a wrong thing to do.

			Attila Kinali

PS: If you need a simple uC and non-volatile memory for a project,
I'd rather use an MSP430FR instead of an AVR with some EEPROM/FeRAM chip.
Makes things a lot simpler if you don't need to access an external
chip but can just use an internal memory cell...beside the uC being 16bit
instead of 8bit (makes software easier in general).

You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor

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