If the stepper ISR takes too long, chars are lost which leads to serial
communication errors like "Line number not +1" or "Wrong checksum". In
worst case, the printer can even do crazy moves.
With this changes, UART interrupts are handled inside the stepper ISR.
This way, no chars should be lost.
lcd_update can take so much time that the block buffer gets drained if
there are only short segments. This leads to jerky printer movements for
example in circles and a bad print quality.
This change implements a simple check: Only if the block currently
executed is long enough, run lcd_update.
This also means the printer will not show actual values on the LCD nor
will it respond to buttons pressed. A option that keeps the menu
accessible is also available.
Aditionaly, slow down if a block would be so fast that adding a new
block to the buffer would take more time. In this case, the buffer would
drain until it's empty in worst case.
If ENDSTOP_INTERRUPTS_FEATURE is enabled this tries to set up interrupt routines
for all used endstop pins. If this worked without errors, `endstops.update()` is called
only if one of the endstops changed its state.
The new interrupt routines do not really check the endstops and react upon them. All what they
do, is to set a flag if it makes sense to call the endstop test we are used to.
This can be used on:
* ARM (DUE) based boards - all pins can raise interrupts,
* RAMPS - all 6 endstop pins plus some other on EXT-2 can raise interrupts,
* RAMPS based boards - as long the designers did not change the pins for the endstops or at least left enough,
* all boards, if there are enough pins that can raise interrupts, and you are willing/able to swap with pins dedicated to other purpose.
planner.h:
fan speed is used to set integer variables, so no need for long.
Basicaly a byte should be enough for all the fan things, as it's 0-255?
planner.cpp:
Save some float multiplications.
We could squeeze out even more by defining feedrate_percentage,
saved_feedrate_percentage and flow_percentage as float instead of int.
Everytime they are used in the time-critical planner, they are casted to
float and multiplied by 0.01. Not done jet, as they are used in LCD menu
functions I don't know well enough.
