Re: [Emc-developers] Raspberry Pi running EMC ???

[Karl S. <ka...@xt...>]

On 07/20/2012 09:38 PM, andy pugh wrote:
> On 21 July 2012 02:47, Charles Steinkuehler<ch...@st...>  wrote:
>> My understanding is that one of the first things the servo thread does
>> when it wakes up is poll for the current position from the appropriate
>> lower-level (software step generator, mesa hardware, or whatnot).
>
> It is, but I think that the problem is that hardware frequency
> generators don't count their pulses.

All depends on the hardware used and how it is configured - there is no reason this "bit of 
electronics" can't support position-counter registers.

Depending on the uP chip - they can have lots of pulse generators - each counter has around a dozen 
config registers - they can be set up to count on the pulse outputs from one of the other counters 
that is set up to generate the pulses - or you can set it up to count pulses via interrupts.  (The 
ARM cortex (thumb ) processors have very fast Interrupts.)

Or using a cheap FPGA - the counter register is just more logic.

The problem of doing this on a PC, is that the code running is not deterministic in time - the same 
instruction can take different amounts of time depending on what is going on, not only on the system 
level, but even on the micro-code level.

Single chip uP systems are much more deterministic for doing real-time work - (even these can have 
differing instruction times - the time to read a flash location can sometimes take an extra cycle - 
but there are ways to work around this. )

In my mind, a single chip uP system, with just a small assembly program, could do the pulse 
generation busy work and make the position information available to the PC-system with time left 
over to play an audio stream.  The question is if there is going to be a latency issue of bringing 
that position information back into the PC without any problematic latency - ( the outer loop is 
running much slower - so I doubt there is an issue. )




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Karl Schmidt                                  EMail Ka...@xt...
Transtronics, Inc.                              WEB http://secure.transtronics.com
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Any cat would tell you that you can only wash one paw at a time;
while we try to do everything at once. -kps

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