I'm doing something like this but for another non-MK related project. I want to capture the count associated with an optical encoder and the worst case is the encoder is sending "edges" every 30 micro seconds. The solution is easy if you use a hardware counter. You gate the counter to a latch. I think if something is changing very quickly and you care a lot about accuracy you need to do the work "logically close" to the encoder, not four processing steps later. With MK most people are using either FPGA based mesa card (which is the VERY best place to do this) or the PRU in the Begal Board. If yu wait until the cost is inside the Linux machine the hardware counter may have crossed another transition. Of course the down side of doing this at a low level is you need to program the device. FPGA programming has a bit of a learning curve and and PRU requires some study too.
( method does not apply here. I keep the count inside a hardware timer/counter register in an ARM-M micro controller.) In general doing this at the lowest level possible is best but like harder to do. The FPGA is the best place On Fri, May 18, 2018 at 9:10 PM, Gene Heskett <ghesk...@shentel.net> wrote: > On Friday 18 May 2018 22:15:37 Jon Elson wrote: > > > On 05/18/2018 07:57 AM, Maxime Lemonnier wrote: > > > Well, from the halscope capture, the index signal looks quite clean. > > > it is digitally generated anyways. > > > > That is misleading, it is only sampled by the PPMC driver > > 1000 times a second, and is also sampled at > > the encoder filter clock rate (1 - 10 MHz, selectable) by > > the PWM controller board. But, if you don't get any > > triggers except at the rate you are sending them, then maybe > > it is fine. > > > > Jon > > Jon is correct there. The sample rate of the halscope is the thread rate > its running on. usually the 1 kilohertz floating point > capable "servo-thread". Its incapable of registering or showing > something that may change state at 500+ kilohertz. > > So this, to get an idea of whats going on, needs at least a dual trace > triggered scope with a 100mhz bandwidth, or a digital sampler with a > similar bandwidth. I have both. Likewise, the triggering logic being > discussed here, also needs to be real, external IC's on a circuit board, > the logic bits and pieces for linuxcnc are tied to the thread they are > addf'd to in the hal file, with a maximum state change at the 1000 hz > servo-thread rate. The external logic can generally keep up with state > changes at a 20+ megacycle rate for the $0.25 parts when you buy them in > tubes of 25 or so. > > But this is an interesting project, so please Maxime, keep us posted on > progress and problems. > > -- > Cheers, Gene Heskett > -- > "There are four boxes to be used in defense of liberty: > soap, ballot, jury, and ammo. Please use in that order." > -Ed Howdershelt (Author) > Genes Web page <http://geneslinuxbox.net:6309/gene> > > ------------------------------------------------------------ > ------------------ > Check out the vibrant tech community on one of the world's most > engaging tech sites, Slashdot.org! http://sdm.link/slashdot > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users > -- Chris Albertson Redondo Beach, California ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
