Post by thread:Re: [PIC] Detect PWM from an RC Servo

Using the CCP module to measure the time between the rising edge and falling edge comes to mind. On 1/13/06, Padu <spam_OUTpaduTakeThisOuTmerlotti.com> wrote: > > Given a hobby RF receiver, the one that is used to remote control RC Cars, > and knowing that they output regular RC Servo PWM (20ms pulse train with > signal HIGH varying from 1 to 2 ms), what is a good method for detecting > this HIGH interval on a PIC18F452? > > Cheers > > Padu > -

> Given a hobby RF receiver, the one that is used to remote control RC Cars, > and knowing that they output regular RC Servo PWM (20ms pulse train with > signal HIGH varying from 1 to 2 ms), what is a good method for detecting > this HIGH interval on a PIC18F452? > Hi Padu, I've used the Capture module. If you want an example in C for the 16F876 (I know that's not a 18F452, but...): http://bobblick.com/techref/projects/sv2hb/sv2hb.html Cheerful regards, Bob

From: "Bob Blick" <.....bblickKILLspam@spam@sonic.net> >> Given a hobby RF receiver, the one that is used to remote control RC >> Cars, >> and knowing that they output regular RC Servo PWM (20ms pulse train with >> signal HIGH varying from 1 to 2 ms), what is a good method for detecting >> this HIGH interval on a PIC18F452? >> > > Hi Padu, > > I've used the Capture module. If you want an example in C for the 16F876 > (I know that's not a 18F452, but...): > > http://bobblick.com/techref/projects/sv2hb/sv2hb.html > > Cheerful regards, > > Bob Thanks for the example Bob, I don't know why but I had in my mind that the CCP module didn't have enough resolution for this type of signal. I'll check your code and the datasheet again. Thanks Padu

Padu wrote: > Given a hobby RF receiver, the one that is used to remote control RC > Cars, and knowing that they output regular RC Servo PWM (20ms pulse > train with signal HIGH varying from 1 to 2 ms), what is a good method > for detecting this HIGH interval on a PIC18F452? The easiest way is to use a CCP module in capture mode. It can only be configured to capture one edge polarity at a time, but this isn't a problem because you have at least 1mS between the leading and trailing edges. Set it up to capture the leading edge, then when you get that interrupt save away the capture value and reconfigure it for the trailing edge. You've got a whole mS to do this, so that should be no problem. ****************************************************************** Embed Inc, Littleton Massachusetts, (978) 742-9014. #1 PIC consultant in 2004 program year. http://www.embedinc.com/products

Padu wrote: > Thanks for the example Bob, I don't know why but I had in my mind > that the CCP module didn't have enough resolution for this type of > signal. The CCP module can capture edges to instruction clock resolution. With a 18F the instruction period is 100nS, meaning it can resolve the 1-2mS pulse width to 1 part in 5000. Other sources of error probably overshadow that by a large margin. ****************************************************************** Embed Inc, Littleton Massachusetts, (978) 742-9014. #1 PIC consultant in 2004 program year. http://www.embedinc.com/products

>-----Original Message----- >From: piclist-bouncesKILLspammit.edu [.....piclist-bouncesKILLspam.....mit.edu] >Sent: 14 January 2006 15:28 >To: Microcontroller discussion list - Public. >Subject: Re: [PIC] Detect PWM from an RC Servo > > >Padu wrote: >> Thanks for the example Bob, I don't know why but I had in my >mind that >> the CCP module didn't have enough resolution for this type of signal. > >The CCP module can capture edges to instruction clock >resolution. With a 18F the instruction period is 100nS, >meaning it can resolve the 1-2mS pulse width to 1 part in >5000. Other sources of error probably overshadow that by a >large margin. Cheap RC servos can not repeatably resolve even 1 part in 256 (i.e. 8 bit) IME due to gear backlash and slop in bearings etc. Regards Mike ======================================================================= This e-mail is intended for the person it is addressed to only. The information contained in it may be confidential and/or protected by law. If you are not the intended recipient of this message, you must not make any use of this information, or copy or show it to any person. Please contact us immediately to tell us that you have received this e-mail, and return the original to us. Any use, forwarding, printing or copying of this message is strictly prohibited. No part of this message can be considered a request for goods or services. =======================================================================

From: "Olin Lathrop" > The CCP module can capture edges to instruction clock resolution. With a > 18F the instruction period is 100nS, meaning it can resolve the 1-2mS > pulse > width to 1 part in 5000. Other sources of error probably overshadow that > by > a large margin. Thanks everybody for your comments... I did my test program and it works perfectly. It wasn't as painfull as I had imagined. Cheers Padu