FRAM71B
User’s Manual
FRAM71B 1.00
- 32 -
©Hans Brueggemann, 2016
17
A
Note
on
FRAM
Technology
FRAM71B uses a memory chip based on FRAM-technology. The advantage of FRAM over
SRAM is (among others) the capability to retain memory contents without a power supply to
the FRAM. Despite this fact, your HP-71B will still greet you with a MEMORY LOST message if
you remove its power supply for a sufficient amount of time. This is not indicating a fault in
your FRAM71B module, but is due to the fact that the HP-71B "remembers" a power supply
loss. When it starts up again, it assumes that its RAM contents possibly got compromised
and clears all RAM to a default state. However, FRAM71B will indeed preserve all contents
that are located in either
•
SYSRAM (F_0x00000 – F_0x1FFFF),
•
CONFIG area (F_0x2C000 – F_0x2C01F),
•
Independent RAM (RAM that you FREE PORTed),
•
Unconfigured FRAM (FRAM areas that are not "mentioned" in the CONFIG area) ,
•
Chips that are configured as ROM.
17.1 FRAM Speed
Compared to SRAM technology that was available at the time the HP-71B was developed,
today's FRAM can have much higher read- and write- speeds. However, your HP-71B uses
fixed memory access timings, so there is no speed gain when using FRAM71B.
17.2 FRAM Access Endurance
FRAM71B uses Fujitsu MB85R4M2T FRAM that has a read/write cycle endurance of 2 x 10
13
8-bit cycles. FRAM71B has to convert each nibble-write cycle of the HP-71B into two byte-
wide access cycles (read-modify-write) for the FRAM.
The standard clock speed of your HP-71B is approx. 640 kHz, which results in a maximum of
470 x 10
3
FRAM-byte-accesses (FBA) per second. This is the highest sustained access speed
that the HP-71B can reach. Thus, your HP-71B's minimum operating time to wear out the
FRAM is T = 2 x 10
13
cycles / 470 kFBA / sec = 11820 hours.
