The target here is to update the screens of graphical and char base
displays as fast as possible, without draining the planner buffer too much.
For that measure the time it takes to draw and transfer one
(partial) screen to the display. Build a max. value from that.
Because ther can be large differences, depending on how much the display
updates are interrupted, the max value is decreased by one ms/s. This way
it can shrink again.
On the other side we keep track on how much time it takes to empty the
planner buffer.
Now we draw the next (partial) display update only then, when we do not
drain the planner buffer to much. We draw only when the time in the
buffer is two times larger than a update takes, or the buffer is empty anyway.
When we have begun to draw a screen we do not wait until the next 100ms
time slot comes. We draw the next partial screen as fast as possible, but
give the system a chance to refill the buffers a bit.
When we see, during drawing a screen, the screen contend has changed,
we stop the current draw and begin to draw the new content from the top.
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.
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.
The extrusion speed was wrong due to a not high enough precision of
esteps to XY steps, therefore now the target float values are used to
calculate the ratio between XY movement and extrusion speed.
The e_speed_multiplier8 was replaced by an absolute multiplier called
abs_adv_steps_multiplier8, therefore one multiplication and bitshift can
be saved inside the stepper ISR. Due to this, also extruder_advance_k is
better suited inside the planner and not the stepper files any more.
.) long to int: Extruder stalls at 10kHz / 20kHz step limits with long.
.) Take the delta_adv_steps calculation out of the step_loops loop. Wasted calculation performance if done inside.
.) >> 2 replaced by 3: Is divide by 8. Reason: Timer 0 runs at 16/8=2MHz, Timer 1 at 16/64=0.25MHz. ==> 2/0.25=8.
