Large Array

[AK]

Hello,

I'm developing an instrument that logs user input in 2 seperate arrays that will be two-dimensional char arrays (appromite size [650][8]). Can this be done without using external memory? What would be the best way to do this? Is there any PIC DIP chips that I can use to do this?

Thanks,
AK

[mpfj]

Basically, no.

You need 2 x 650 x 8 = 10,400 bytes.

The biggest RAM size in any currently available PIC is 3968 bytes.

You will have to use some external storage. If speed isn't too high, I would suggest Ramtron's FRAM devices.

[AK]

Thanks for the input. Is there any sample PIC code to interface an FRAM chip?

AK

[mpfj]

It just uses standard I2C or SPI as per most serial eeprom devices.

See the EX_EXTEE.C example.

[arunb]

Hi,

You could also use an EEPROM, Microchip has several of them, both I2C and spi..eg.- 24LC256 , 24LC512 etc.

thanks
arunb

[mpfj]

[AK]

Is there a specific FRAM chip that you would recommend for my application?

[newguy]

I've been playing with a ramtron FM24C64 for the past couple of weeks. If you've ever used a serial EEPROM, this thing is a drop-in replacement. It works well, and it's very nice not having to worry about the very slow write cycle common with ordinary eeproms.

As for recommendations, the simple answer is to get one large enough for your application. If you need a DIP package for prototyping/playing it seems that ramtron no longer makes most of their serial FRAMs in DIP packages - but Newark still had plenty in stock.

[mpfj]

[AK]

Will I be able to use a single chip? Sorry, I don't have much experience with external memory. Thanks

[mpfj]

Almost certainly, yes.

Any PIC device containing an MSSP module can support either SPI or I2C.

The difference between SPI and I2C comes down to number of pins required to communicate, speed and power requirements.

I2C

2 wire system (clock and data)
1MHz max bus speed
current ~ 1mA

SPI

3 or 4 wire system (clock, data in, data out, chip select (only in multi-chip setups .. i.e. 2 or more SPI memory devices))
25MHz max bus speed
current ~ 3 to 10mA

Since SPI has seperate data lines, it is bi-directional, which is why is can run faster.

So your choice of device rests on your requirements, i.e. How often is your instrument having to log user input ? How much power can you spare ? Which PIC device are you using ? ... etc ...

Check the Ramtron datasheets, and then get some samples (via the Ramtron website).

[AK]

Thanks for the help. After looking at the parts on the website, I believe the FM24C256 will work for my specs (see first post). Do you see any problems with this?

[mpfj]

Should give you plenty of room for expansion.

Just note that the FM24C256 has *no* 3volt alternative, unlike the SPI version

[AK]

Thanks for the help

[Guest]

How will the FM24C256 have enough memory for this application? It has only 256 kbit.. I think he picked the wrong chip.

Bill