write focuser position to eeprom

[frnhr]

Problem:

Each time when focuser looses power (i.e. when powered on) we need to re-home it. This feels like an unnecessary step and is easily forgotten.

Possibility 1: Use relative values

Just use "0" as start value and go to + or -.

This would work fine except that MaxIm DL sometimes freaks out on negative values of focuser position. E.g. cannot perform auto-focus.

Also, working with absolute values it more intuitive for the user.

Possibility 2: Save absolute values

Arduino has EEPROM memory, a natural place for storing this value.

There is a guarantee of 100k write cycles. If we write only end position of moves, this gives us a few decades of use:

100000 / 10 refocus_per_night / 20 nights_per_month / 12
 months_per_year =~ 41 year

Implementation:

• Store position in EEPROM when done moving
• Do not store position while moving (this would reduce EEPROM life to single-digit number of years)
• Store a sentinel ("magic") value when movement starts
• When get command to move, and EEPROM contains the sentinel value, automatically re-home
• Add "rehome" command to CLI