Clarify that the LDO is the adjustable flavor

[braddr]

I know this is an old project, but it solves a useful problem.

The 1117 ldo comes in a number of flavors, but which one isn't well specified. As designed, this circuit seems to be looking for the adjustable version, based on the use of the resistor divider on pin 1 (ADJ) rather than straight to ground. That said, the fram chip wants a 3.3v Vdd, and not the 3.9 that it gets with the R1 and R2 values used. The 3.9 is above the allowed range (2.0-3.6) . Why not remove the divider and just use the 3.3 specific flavor? Or swap the 470 out with a 360 to get the desired 3.3.

Am I missing something?

[Qw3rtzuiop]

I used the adjustable Version. The logic voltage of the Keithley is 5V. Maximum supply voltage of the FM28V100 is 4,5V. The higher supply voltage of the FM28V100 is chosen to be closer to the digital input voltage spec and to be sure that the Keithley is accepting the digital output high signal.

[braddr]

Hrm.. According to the datasheet:
https://www.infineon.com/dgdl/Infineon-FM28V100_1_MBIT_(128K_X_8)_F-RAM_MEMORY-DataSheet-v08_00-EN.pdf?fileId=8ac78c8c7d0d8da4017d0ec8921d410d

VDD Power supply voltage minimum: 2.0, typical: 3.3, maximum: 3.6 V

Your reasoning makes sense in terms of managing the threshold for a logical 1, but I'm concerned about running the chip over the max spec'ed voltage. That seems to strongly suggest that a level shifter for the data and address pins is called for. Of course, that adds cost and layout difficulties.

[Qw3rtzuiop]

A level shifter would be a better solution to comply with all the specs. However it is not needed as you can see from the linked eevblog post and my experience. In the linked datasheet are the maximum ratings on top of page 6. Its 4,5V.

[braddr]

Apologies for not following up sooner. I installed my FM28V100 backwards and that killed the chip (my own fault for not double checking the orientation). In the mean while, I discovered this chip: SST39SF040. It's a compatible drop in replacement chip for the DS1245Y. And even better, it's only $3.99 in quantity 1 from digikey. I ordered one and have been running it in my Keithley 2001 for a short period of time and it seems to be working fine. So much cheaper and easier than the fram or nv ram options.

[Qw3rtzuiop]

Wow that is very nice. I wasnt aware of the compatibility of the SST39SF040.