[Oberon] Flash wear levelling

[Paul Reed]

Hi Wojtek,

> ... The other pages present my
> thought and investigations on the Nordic radio module and Oberon
> software, and some general remarks. All these pages are part of the
> RiskZero Rev 1 schematic.

Just some experience regarding your section on "Flash Chip Notes", where 
you point out that SPI flash could be used instead of an SD-card:

The Oberon file system assumes a disk device of infinite endurance. As 
I'm sure you're aware but others may not be, flash is a technology which 
has a limited endurance (~10K-100K erase cycles), hence the SD-card was 
chosen over SPI flash because it internally implements wear levelling, 
which increases the effective endurance, especially of the root 
directory sector.

For the System Construction class last year, we used on-board SPI flash 
for the disk instead of the SD-cards used in most previous years, 
because it was cheaper and more convenient, and the low amount of use 
meant that wear was essentially irrelevant. (I also did this on the 
Spartan 3 board Xilinx Platform Flash one of the first years we used the 
FPGA Oberon system.)

I suppose you could have cheap+endurance by using a PDIP package for the 
SPI flash and socket it, replacing it when it wears out, but then you 
have the price of the socket, the through-holes and the larger footprint 
(which really look almost comically huge these days on a modern 
surface-mount-technology design), the decreasing availability of such 
packages, etc.

Cheers,
Paul