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Gary Cray · 07-16-2017, 06:47 PM

How can FRAM work faster than SRAM on the HP48 ? Are there any software configurable wait states in RAM access ? (@JDW#32)
FRAM 75ns access time (MB85R4M2T) but read cycle time 150ns (equals write cycle time)
SRAM normally 70ns to 150ns access time (usually <120ns)

I doubt FRAM will (or can) make the HP48 faster. (like matthiaspaul#38 said)

Flash is less reliable than FRAM ? Flash is normally used as (program) ROM with practically unlimited amount of read cycles (within the 20 years of data retention). Flash write access is also sector based (at least 512 bytes) and therefore (and because of other limitations) flash is not suitable for RAM replacement as working memory. SSD flash disks (JDW#14) employ a special wear-out leveling method to redirect frequently write access to the same data block to another flash sector. Thereby achieving more sector-write-cycles till it wears out and loss of data is imminent. FRAM could work fine in a SSD (speeding up the write access) but would make things really expensive with the silicon technology currently used.
The use of EEPROM instead of RAM for working memory is likewise impossible, 'cause no read access is possible immediately after write access. You would have to wait ~1ms after writing one byte or a whole page, before you can read any data or write another page (16 bytes).

While the above are only thoughts of mind, my real question is: does it make any sense to interface a microSD card to the HP48 ? (@matthiaspaul#45)
You would have to use a super-cap to supply enough power to the SD card. The super-cap would have to be charged 40..60 seconds at <20mA/Vcc(on) to store up enough power to achieve a 10second write access at approximately 100mA. Also charging a 3,3F/3V super-cap to 2,0V (2,5V at max.!) using the Vcc(on) can take some time (I guess 70..100s). Super-caps like rechargeable batteries are listed with a few 1000 operating hours but also 500000 cycles and at least 2years of shelf life.
https://www.vina.co.kr/winko/data/produc...335_QG.pdf
The caps are bulky and keep their charge a couple of days (better discharge before storing away a module with the super-cap). To speed up things, you also would need some MCU (MSP430) to interface the SDcard. The SDcard needs 3,3V logic (like the MSP) while all the rest is 5V logic. You would need 5V level translators, some step-down and one step-up convertor like TPS54062/TPS62120, TPS61071 … and a lot of software!

Would this make any sense “doing it, just because we can” ? Anybody who needs an SDcard already has an HP50. While the FRAM card seller at least made a “little” profit (although many would rather buy the cheaper RAM cards), the SD card interface would really be no “bargain”.

Gary
PS: replacing the 1,5mm double layer FR4 PCB by 2mm is a matter of price. I would prefer gluing some 0,2mm FR4 on the back side of the connector or using 100um copper if nessesary.