HP-48 FRAM Cards

[Chris Dreher]

Short answer
I can't answer about the longevity of FRAM cards that get a lot of reads, but in short I suspect you don't have to worry too much about it.

Long answer
One way to avoid excessive reads and writes to a FRAM card is to not merge it with main memory (i.e. don't use the MERGE or MERG1 commands). You can only merge 128KB or smaller cards on the SX slots 1 and 2 or the GX slot 1. Any card that is 256KB or larger can only go into GX slot 2 which is an unmergable slot.

If you don't merge the FRAM cards, then they just act like a simple storage device (vaguely similar to a USB FLASH drive). As such, they simply don't get read that much by the calculator.

If you do merge the FRAM cards (again, must be 128KB or smaller), they will get read more often compared to an unmerged card since they extend the internal RAM of the calculator. Even then, I wouldn't worry too much about it. The calculator reads the built-in RAM far more often than a merged RAM or FRAM card. A merged FRAM card will be used to store objects in port 0, your home directory, the stack, and maybe the temporary objects when they pile high enough (the pile of temp objects starts out in built-in RAM). However these things are read far less often than the internal structures kept in built-in RAM (ex: the screen buffer is kept in built-in RAM).

[EugeneNine]

Did anyone ever builds one? I would like an expansion card but the ebay prices are a bit too high.

[Hans Brueggemann]

(03-23-2017 02:45 PM)Han Wrote:  I still am not quite clear on the read/write cycles. While there may be more than 10 billion times the number of cycles as FLASH (according to an earlier post), what exactly happens during a read cycle? Does the entire content of FRAM get re-written? Or just the bits that got read? What happens when FRAM gets mapped to parts that get read frequently? For example, if the display data is mapped onto FRAM, then there are lots of read/write cycles without the user ever doing anything but having their calculator on. Can someone comment on this?

you can do the math, here the excerpt from FRAM71B's (a memory extension for the HP-71B) user manual concerning FRAM endurance, as an example:

"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 103 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 1013 cycles / 470 kFBA / sec = 11820 hours."

hope that helps

[JDW]

(03-23-2017 04:58 PM)EugeneNine Wrote:  I would like an expansion card but the ebay prices are a bit too high.

HP calcs aren't a poor man's hobby! :-)

Just do like I did a few months ago. Bite your tongue. Go to EBAY. Click BUY IT NOW. It does a calculator good!

(I have a 128k FRAM card merged in my 48GX and currently am using Meta Kernel with it. No problems so far.)

[EugeneNine]

Too many hobbies I guess, spreads the $ too thin.

I keep thinking though I should be able to order a ram chip from mouser or digikey and whip up a PCB in kicad and be good.

[Gary Cray]

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.