data storage

ferrousduke · Joined: 31 Mar 2008 Posts: 29 Location: Mumbai, India

hello,
I have to store the value of the analog channel into the mc memory. anyone can help ? data appears at AN1 with sampling rate of 1 ms. i need to store that value instantaneously.

any basic commands for that?

Matro · Guest

Which memory?

The EEPROM or program memory will never be fast enough to allow a write every 1ms.

Matro.

ferrousduke · Joined: 31 Mar 2008 Posts: 29 Location: Mumbai, India

EEPROM. I guess I have to slow down the process. I dont want it more than 30 ms. how fast can it write into memory ?

Matro · Guest

It depends on your chip and the Xtal frequency you are using.

The best is to perform a test :
- generate a rising edge on a spare pin
- run the write_eeprom() command
- generate a falling edge on the same spare pin

You just have to look at the pin with a scope to know how long it takes to write in EEPROM.
Take 1.5x or 2x this time to be sure.

A good thing could be to use asynchronous write. So the chip can continue processing data during the EEPROM writes :

Humberto · Posted: Wed Apr 16, 2008 7:24 am

A FRAM would be your best choise.
A FRAM internally has a structure similar to a DRAM (dynamic RAM), but the storage of
data is made by means of a ferroelectric effect, by opposition to the conservation
of the load in the intrinsic capacitor in a dynamic RAM of floating gate.
Its main features:
- very fast access time.
- virtually limitless writings.
- data retention without energy consumption.

You will search for some application in this forum.

Humberto

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

Also note that EEPROM has a finite number of write cycles before it fails. The PIC I am using is rated for 1 million write cycles. If you were to write the same location every 1ms you would wear it out in 1000 seconds, about 17 minutes.
_________________
The search for better is endless. Instead simply find very good and get the job done.

Matro · Guest

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

ferrousduke · Joined: 31 Mar 2008 Posts: 29 Location: Mumbai, India

I m using 18f4550 with crystal 20 Mhz. I have this position sensor from Renishaw attached to AN1.. (just an assumption) data in this terms would be a single axis coordinate. but I want to store that particular coordinate associating with XY coordinates, so the set of data comprises of 3 coordinates.. (X-Y-Z).. XY will be simply incremented at particular cycles. Z coordinate I need to sample. Moreover, the scanning should take lot of time which means gathering of data will be for long time and I need to save it temporarily in my controller till my USB talks to it and then I can transfer the file to PC. I hope I put myself in correct words..

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

We are trying to determine what type of memory is needed. For example, the easiest to use memory is the internal RAM but this will loose it's contents when power is removed. EEPROM doesn't loose it's contents but has other problems.

You already told us the data is coming in at a rate of 1 sample each 30ms, and if possible you would like to speed this up to 1 sample/ms.

To give a good suggestion we need an answer to the following questions: