On Wed, Sep 03, 2008 at 12:24:00AM -0600, [EMAIL PROTECTED] wrote:
> i just interesting, is pulse per revolution same as steps per revolution
> (x something to get steps per inch)? can i say that 1 step/pulse = 0.3125
> nanoseconds and when it comes close to 12500-13000 is is too fast?
> thanks
Yes, there is a very clear relationship between BASE_PERIOD and the
maximum number of pulses that can be created on a dumb I/O pin.
A dumb I/O pin can have no more than one transition (e.g., from LOW to
HIGH or HIGH to LOW) per BASE_PERIOD.
A "step pulse" consists of two transitions: LOW to HIGH, followed by
HIGH to LOW (assuming an active HIGH step pulse). Therefore it takes
two BASE_PERIODs.
So for a BASE_PERIOD 'b' nanoseconds, the maximum step rate is 2*b
nanoseconds, and the maximum step frequency is '1e9/(2*b)' Hz.
Google can also find this number for you, if you enter the search
1/(2*14000 ns) in Hz
that's different than the '1e9/2*b' formula I showed above, because in
the first case I accounted for the "nano" prefix by hand (nano=1e-9,
1/nano=1e9) and in the second case google accounted for it
automatically.
You can also find this in the unix program 'units' (which you can
install by selecting the package 'units' in synaptic):
$ units '1/(2*14000ns)' Hz
* 35714.286
(As an aside, I can't recommend these two tools enough when it comes to
performing computations on real-world quantities. Particularly units,
since it automatically carries units through the computation and
verifies that they match the desired result units:
$ units '(5V)^2/1 ohm' 'A' # Error: quantity is actually in watts
conformability error
25 kg m^2 / s^3
1 A
)
When emc uses smart hardware, this relationship between BASE_PERIOD and
the maximum number of transitions on an output pin is broken. For
instance, Jon Elson specifies that his step generation cards can
generate step waveforms up to 5MHz (limited to 300kHz at the board
terminals, presumably due to filtering, and/or isolation designed into
the board). That rate can be achieved regardless of emc's BASE_PERIOD.
Jeff
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