Data-Logger

[barakorit]

Hello guys,
I need a circuit of data-logger which measure temperature (it samples the value of a resistor that changes with the temperature every minute) and saves the results in it's own inner memory for a maximum period of a year.
It should have a usb connector and no CD would be added to the product, so the software should be in the inner microcontroller.
I don't need the software, but only ideas for the hardware.
Any help would be a life saver.
Thank you very much.

 

[Ubergeek63]

why usb? it adds a lot to the complexity.

A micro can easily access an MMD card, just swap it out when you visit site installation or pop it out and read it and pop it back in.

 

[blueroomelectronics]

How many 8bit samples do you want to collect.
An 18F2550 has USB support, A/D and I2C/SPI

 

[barakorit]

The device has to sample the temperature every minute and save the data for a year.

 

[Pommie]

The device has to sample the temperature every minute and save the data for a year.

To sample 10 bits of data every minute for 1 year requires around 5M bit of storage. Using a 18F2550 and eight 1 meg serial EEPROMS (on 2 select lines) will handle what you require but I would go with Ubergeek's solution and use a swappable memory card.

If you also want to store date and time information then the memory requirement increases.

Mike.

 

[ericgibbs]

hi,
As Mike states, the problem is date/time tagging of the samples, takes a lot of memory.

A technique I use was to store the date/time when the log button was pressed,
then start saving data samples once every second.
At midnight the date/time was saved again in the next available memory location.

The best I have ever achieved using a software clock from the local xtal is about 1 minute/month,
the xtal osc is trimmed for best accuracy.

If you did store the date/time and the data for a full year, the download via the RS232 is slow.
During the download the 'tag' date/time was calculated from the start date/time, resynching at the midnight tag.

The other factor is setting the date and time, the method I used was via the RS232, uploading the PC's date/time.

Hope this helps.

 

[gramo]

Definitely won't be an easy task if your not familiar with micro's

I'm not sure if you wanted a remote logger or a local one that is permanently connected to the PC, but the previous posts kind of suggest that it is a remote device

**broken link removed** (time & date IC) with your micro would probably give a lot better results, but its dependent on the quality of the crystal and the circuit layout. 1 minute a month is definitely achievable as ericgibbs has managed

**broken link removed**

From there you can get time and date stamps fairly easily. Storing this data into EEPROMs is the next hassle, given the amount of data, the PIC's on board memory won't be suffice..

**broken link removed**

**broken link removed** of using multiple EEPROM's with I2C. Each device contains 32K x 8 (256 Kbit) of memory.

Each data packet would contain: Hour, Minute, Month, Day, Year, Sample information. Given that all other samples except the Sample are bytes (sample would most probably be a Word result - 16bits) then each packet of info would be 7 Bytes.

A single 32Kbyte eeprom could store 4571 tagged samples, but in a year there are 365 * 24 * 60 minutes (525600).

One solution would be to use a more expensive data storage solution (bigger eeprom), another is to refine your method of storing data, eg, only store the Time/Date at the start of each EEPROM in use, from there each sequential stored sample is 1 minute from the last... Your "downloading" software on the PC would recognize this and know how to treat the following information - greatly improving your storage capabilities.

Perhaps a mix of both methods would suite your application

 

[blueroomelectronics]

The 18F452 has no internal osc, you could use the 32kHz output of the DS1307