Page 245 - Embedded Microprocessor Systems Real World Design

These protocols can be combined. For instance, every command might be
acknowledged when received, but an execution acknowledge is sent only if there
is an error.
Any scheme that does not force the higher-level controller to wait for acknowl-
edge of execution before sending additional commands must have a mechanism
to handle errors. If an execution error occurs in command A, but commands B
and C have already been sent, how does the higher-level controller know which
command did not execute? What if command C depends on command A execut-
ing correctly? For example, our robotic arm might be told to move to a certain
position (command A), insert a tool in a slot (command B), and turn it (command
C). If the first command did not execute, then the last one is pointless and may
even cause damage. So the protocol needs to speciQ what happens in case of an
error. If commands can be “pipelined” (a new command sent before the old one
is executed), you need to stipulate how many commands can be allowed to stack
up so that the buffers do not overflow.

Design Piifalls

Multiple Measurements Be careful of having two processors measure one thing.
Because the “thing,” whatever it is, will be measured with a digital system, there
always is the possibility that the two processors will get different results. If they are
measuring time, there will be at least one clock ambiguity in the measurement. If
they are measuring a voltage, there always will be an ambiguity of at least one count
in the two ADC output results. This can be a problem if there are fixed thresholds.
For example, if you are moving wooden blocks down a conveyer system and one
processor determines that the length of a block is just barely too long, be sure
another processor will not declare it to be OK The first processer might skip
sending the block to a planing process, while the second one proceeds with some
other process that depends on a smooth surface. If there are fixed thresholds
for what you measure (too short, too long, too heavy, voltage too high, and so on)
be sure that the first processor that detects an error overrides the measurements
of all subsequent processors. Or else be sure that a conflict does not cause
problems later.

Synchronization Say you have a process controlled by multiple processors, like
the wooden block example just mentioned. One processor cuts the blocks to size,
the next one planes them smooth, the third one stamps a logo on the blocks, and
so on. Say that everything in the system is synchronized to a clock that occurs once
each time the conveyer system moves 0.1 inch. If data are passed between proces-
sors as each block moves between the regions controlled by each processor, there
is a risk of a one-clock ambiguity in the timing. Be sure these cannot add up as
the blocks move along. Either keep the time increment small enough that the

226 Embedded Macroprocessor Systems