Power Down Memory Save

MGVdec2000

I'm designing a very simple motor control panel using a rotary encoder connected to a Prop1. The user selects the desired RPM via the encoder. The Prop1 then outputs to a DAC to generate a 4-20mA control signal to a COTS motor driver.

On powering down, I would like the Propeller to write the current RPM set point to non-volatile memory. That way, the user doesn't have to re-dial the RPM setting from 0 each "day". I'm concerned about writing to the EEPROM too frequently, as I know these are life limited. I thought it might be best to monitor the input voltage and when it drops below a certain value, write to the EEPROM. Of course I would have to have some "hold-up" capacitance and monitor the voltage up stream of that. I'm just having a little trouble picturing it all. Wondering if anyone has gone down this path before and would be willing to share.

Thanks in advance!

Peter Jakacki

Write to the EEPROM whenever you change the setpoint, you will wear out the button/encoder before you even get close to wearing out the EEPROM. You can use a delayed write so that you only write it once. Remember that 1 million writes is the "minimum" and I did and test once and gave up when I got close to 10 million writes.

Just use an RC DAC plus an opamp to generate 4-20ma.

MGVdec2000

That was another option I considered. When the value changes, wait 1-3 seconds after it stops changing and write to memory.

Could you elaborate on the "RC DAC"?

Thanks!

Peter Jakacki

That's what I meant by "delayed write". Trying to only write on power-down requires power-fail detect circuitry, a sense line, a cog and software dedicated to sensing that, and fingers crossed that the power rail remains stable etc etc. Always a bad idea but the only option we had in the bad old days.


Each cog has two counters that can be used in a variety of modes and duty cycle mode (not quite PWM) is suitable for connecting to an RC network to filter the duty cycle pulses from 0...100% duty to a voltage of 0 to 3.3V. You can use this voltage to control a simple current generator circuit using a transistor or opamp. We can generate audio waveforms with this or simply a set-point voltage.

MGVdec2000

Got it. Thanks again!

localroger

Instead of trying to write on power fail, use an IIC or SPI battery RAM or RTC with memory like the DS1307, and write as often as you want.

MGVdec2000

localroger wrote: »

Instead of trying to write on power fail, use an IIC or SPI battery RAM or RTC with memory like the DS1307, and write as often as you want.

That's a good suggestion as well. I had considered a current date/time display, but didn't know where to start. I'm using a 4x20 LCD dot matrix screen (New Haven NHD-0420DZ-NSW-BBW) to provide some feedback to the user. Maybe I can add this feature at some point.