Added snapshot of current hardware design

firmware/docs/implementation-notes.adoc

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+Motion control in an interrupt.
+
+= Parts
+This consists of two parts, the planner and the executor.
+
+The planner receives target positions. Each time it receives a target
+position, it replans so as to reach that position as soon as possible;
+the output of a plan consists of a set of motion segments, each
+following a 3rd order polynomial.
+
+The executor processes the current state and decides whether to toggle
+the step lines of the MCU.
+
+These two processes communicate by means of a command queue.
+
+== Executor
+1. Update a cycle counter
+2. Evaluates the next output of the position polynomial (3 adds)
+3. determine whether to toggle a stepper, and do so.
+4. Determine whether to start the next segment; if so, advance in the command queue.
+
+
+
+== Command queue
+
+The command queue takes the form of a ring buffer, with each item
+containing a motion segment.  The ring buffer must be large enough to
+contain a complete motion profile plus a terminating "stop" segment.
+
+A motion profile segment consists of the following values:
+
+* For step computation
+  * Δx1..Δx3 (initial values)
+  * start time (in ticks)
+* For use by the planner
+  * v₀, vₑ: Initial and terminal velocities
+  * a₀, aₑ: Initial and terminal acceleration
+  * The actual position at the start time (written by executor)
+
+The following invariants hold for the command queue:
+
+
+== Planner
+
+=== Aborting
+
+In case of an abort, the fastest stop profile will consist of at most
+3 segments: const -jerk to max -a, const -a to to lead-out, const +j
+to a=v=0. 
+
+If we're already in the last three segments, we're already in
+a race to a stop, so there's no need to handle an abort specially.
+
+In the cv segment, we can simply advance the start times of each leadout segment.
+In the lead-in, we need to replan; this is, however, easy enough:
+
+1. Predict state in $t_plan$ cycles
+2. Identify how to reach a=0; prepare a segment accordingly
+3. Feed the end values from said segment to the leadout calculator
+4. replace the next N commands with this leadout.
+
+
+