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How to store variable values when power is down in PIC

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GreenBeret

New Member
I am writing a count down timer using PIC. I want to store the last set time to a permenant storage so that even after restart i want to get the last set time. How can i do this?
 

Diver300

Well-Known Member
Most Helpful Member
The simple answer is in the EEPROM. Chose a PIC with EEPROM in it and store any data you want.

However, there is a significant problem with that. The EEPROM can only be written to a limited number of times, maybe 1 million times. If you store time every second, you would wear the EEPROM out after two weeks running. What you have to do is store the time only when the power is failing. You have to detect loss of power, but still have enough power remaining to run the PIC for long enough to store what readings you need in EEPROM.

Alternatively you can have a small battery or a big capacitor to keep the PIC going. When you loose power, put the PIC into sleep mode, and wake it when power is restored. Power consumption in sleep mode is virtually nothing, and all the values in RAM are kept.
 

Noggin

Member
You can use a circular buffer too in order to increase the number of times you can write to EEPROM. However, I'm with Diver, I think it is better to write to EEPROM only when you need to do so.
 

Oznog

Active Member
+1 to Diver300's response. Both options are possible, each with the issues he stated.

One more note, writing to EEPROM to make things available if power goes down is touchy. IF the power goes out WHILE the EEPROM is being written, then we don't know what'll be stored. In fact it's somewhat uncertain that it won't write and corrupt a completely different EEPROM location.
 
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