6 Mar
2014
6 Mar
'14
2:29 p.m.
On Thu, Mar 6, 2014 at 11:18 AM, Steve Lafferty <steve at tronola.com> wrote:
On the question of whether F-RAM would be a suitable
replacement for the
CY62256 CMOS/battery solution: There was a longevity calculation posted
that appears to have been in error. The 1e14 cycle limit was said to be
good for 30-years continuous access at a 1us cycle rate. I calculate about
3-years. Perhaps 2us would be a more appropriate cycle rate for the PDP-8
and that would make it 6-years.
The 5V FRAM devices have a lower rated endurance than the 3.3V devices,
10^12 cycles for the FM1808 (5V) vs. 10^16 cycles for the FM18L08 (3.3V) or
10^14 cycles for the FM18W08 (wide voltage range). For the 5V that's only
23 days at continuous 2us cycles.
Each read or write cycle counts against the endurance of a full "row" of
the FRAM, which is four bytes in the case of the FM1808 (5V), unspecified
for the FM18L08, and eight bytes in the case of the FM18W08.
Note also that FRAM parts latch the address on the falling edge of /CE,
unlike static RAMs.