Post by thread:[PIC]: PIC16F84/16F877 ISR SERVO CONTROL

I'm working on a project (currently using the PIC16F84) in which I would like to be able to set the servo (1-8) to a specific setting (1=Extreme left/128=center/255=extreme right -- or whatever) somewhere in the main loop, and have an interrupt take care of actually maintaining that setting in the background. I'm writing this is MPASM/GPASM for speed & compatibility. I've looked at a number of servo control source, and I haven't found anybody else doing it this way (it appears everyone dedicates a 16f84 to this task). I figure this would have to be table driven, but just can't get this to work. The timing is something else I'm having trouble getting just right. Both chips are clock externally w/a 20mhz crystal. I can't believe I am the only person that has ever wanted to do this, does anybody out there have any examples I can look at that perhaps I've missed? I'm not new to programming (just new to PIC's), and it's killing me that I can't figure this out. I've been working it for days now... I can send the code I have currently if anybody is interested.. -- Rob __________________________________________________ Do You Yahoo!? Get personalized email addresses from Yahoo! Mail - only $35 a year! http://personal.mail.yahoo.com/ -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

> > >I can't believe I am the only person that has ever >wanted to do this, does anybody out there have any >examples I can look at that perhaps I've missed? I can't help you with pic code, but my AVR code does exactly that, on eight servos. I use two timers, One is the opsys tick, which does a lot of other things, including scheduling the servos, and starting the output pulse. The other timer is loaded with the output pulse width. When it interrupts, the servo pulse is shut off. You can probably do a similar thing on the F84, except that you don't have the second timer to do the pulse width, so you'd have to do it in some sort of "sit and spin" loop. A warning: Not all servos are satisfied by the typical 1-2mS pulse time. In my code, I have equates for servo types that set the min/max pulse width appropriately, so all the application needs to do is set a servo position byte in ram from 0-255 and the result is always the same position (within reason) even between different servo brands. -- Dave's Engineering Page: http://www.dvanhorn.org Where's dave? http://www.findu.com/cgi-bin/find.cgi?kc6ete-9 -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

>I'm working on a project (currently using the >PIC16F84) in which I would like to be able to set the >servo (1-8) to a specific setting (1=Extreme >left/128=center/255=extreme right -- or whatever) I wrote a program that controls a single servo, adjusting the pulse widths for jitter-free operation at any setting along its range of motion, controlled with a potentiometer input to the A/D. With only 3 locations predefined, it should be fairly simple. What's your input source to determine the servo positions? The only thing you really have to worry about is the resolution. If you're off by ~5us in pulse widths, you're going to jitter back and forth. >figure this would have to be table driven, but just >can't get this to work. The timing is something else At 20MHz, you should be a little better off than I was at 4MHz, as you can tolerate more variation in the instruction cycle counts for pulse timing. At 50Hz cycles to the servo, you could just put the delay time for the high pulse in the table for each of your 3 positions and send the line low for the remainder. As for handling 8 servos simultaneously, you'd have to poll each servo's delay timing to shut it off at the right time. This is where you might run into trouble with jitter. But with 8 servos and 20ms between cycles for each servo, you could pulse one servo, wait for the 1-2ms high pulse, then move on to the next servo. You'd run through all 8 in only 16ms, leaving 4ms for any other calculations you might want to do. At 20MHz, that's quite a few instructions. Or, better yet, you can set up an interrupt to go off at your 1-2ms pulse time and send the line low, if 4ms isn't enough to do whatever you have to do between pulses. If you keep the same values going through the loop (only 3 distinct servo positions), you probably won't have any problem with jitter. You should be able to do this with an '84 without a problem, if you don't want to shell out the expense for the '877. And if any of this advice doesn't seem right, ignore it... I'm having bad luck today :) --Andrew _________________________________________________________________ Get your FREE download of MSN Explorer at http://explorer.msn.com -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

I have done exactly this, but in C, not assembler. I did use some inline asm code in the ISR for better control of the timing. I don't think I can share the source code with you since I did the job for someone else, but I could possibly help you do something similar if you want. In application I use a 16F84 to control the servo and do some sonar ranging and serial communication, all of which uses a little under 50% of the available program space. Dale On Thu, 8 Feb 2001, Robert Shady wrote: {Quote hidden}> I'm working on a project (currently using the > PIC16F84) in which I would like to be able to set the > servo (1-8) to a specific setting (1=Extreme > left/128=center/255=extreme right -- or whatever) > somewhere in the main loop, and have an interrupt take > care of actually maintaining that setting in the > background. I'm writing this is MPASM/GPASM for speed > & compatibility. > > I've looked at a number of servo control source, and I > haven't found anybody else doing it this way (it > appears everyone dedicates a 16f84 to this task). I > figure this would have to be table driven, but just > can't get this to work. The timing is something else > I'm having trouble getting just right. Both chips are > clock externally w/a 20mhz crystal. > > I can't believe I am the only person that has ever > wanted to do this, does anybody out there have any > examples I can look at that perhaps I've missed? > > I'm not new to programming (just new to PIC's), and > it's killing me that I can't figure this out. I've > been working it for days now... I can send the code I > have currently if anybody is interested.. > > -- Rob > > > __________________________________________________ > Do You Yahoo!? > Get personalized email addresses from Yahoo! Mail - only $35 > a year! http://personal.mail.yahoo.com/ > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. > > --- The most exciting phrase to hear in science, the one that heralds new discoveries, is not "Eureka!" (I found it!) but "That's funny ..." -- Isaac Asimov -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

On Thu, 8 Feb 2001, David VanHorn wrote: > A warning: Not all servos are satisfied by the typical 1-2mS pulse time. > In my code, I have equates for servo types that set the min/max pulse width > appropriately, so all the application needs to do is set a servo position > byte in ram from 0-255 and the result is always the same position (within > reason) even between different servo brands. Amen! I switched from an Airtronics servo (my project) to Futaba (someone else's choice) and all the timing went right out the window. The equates are a good idea, David... so you select the servo type at compile/assembly time, or do you use some means of telling the PIC which servo he's got and do a table lookup for the timing? Dale --- The most exciting phrase to hear in science, the one that heralds new discoveries, is not "Eureka!" (I found it!) but "That's funny ..." -- Isaac Asimov -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

> >Amen! I switched from an Airtronics servo (my project) to Futaba (someone >else's choice) and all the timing went right out the window. The equates >are a good idea, David... so you select the servo type at >compile/assembly time, or do you use some means of telling the PIC which >servo he's got and do a table lookup for the timing? In the current code, it's assemble time select. It's on my website, if you're interested, under micros, AVR, near the bottom. -- Dave's Engineering Page: http://www.dvanhorn.org Where's dave? http://www.findu.com/cgi-bin/find.cgi?kc6ete-9 -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Hi Everyone, I created a project which did this for the now (seemingly) defunct Electronics Australia magazine, you can find all the details at http://www.digitalnemesis.com/ash/projects/picservo It is controlled via a bit banged serial connection and the servos are serviced in an interrupt routine. However, after the recent replies, I now know why I couldn't get full travel out of my futaba servos.. I assume 1-2ms as the max range... interesting.. I adjusted by allowing the user to set and "offset" then allow the servo to travel +/-45 degrees from that. It seems that my futaba servos thought that 1-2ms = 90 degree travel, which kind of makes sense when you look at the maximum travel you would get out of something connected to the servo like a rudder or something. Anyway It may give you something to work from. All the ASM code is there and well commented, as well as the (simple) circuit and PCB artwork for the project. Hope that helps! Ash. > {Original Message removed}

Hello Rob, My application uses 2 servo motors, here is what I do and it works well: in a loop: - Set high bits bits that act as signals for the servos: - Wait 1mSec - Set low the bits of the motors that needs to be on one side. - Wait 0.5mSec - Set low the bits of the motors that needs to be on the center. - Wait 0.5mSec - Set low the bits of the motors that needs to be on the other side. - Wait 18mSec. You can have as many motors as you want (as you have enough bits). I use one timer to measure the time. The only important issue is to reset the timer before deciding which bits needs to change, otherwise, the motors might jitter due to pulse width variations. If you do this, it can work with 4MHz and only one timer and not interrupts are needed, so even , a 16c54 could be used. Chaipi At 08:18 08/02/01 -0800, Robert Shady wrote: {Quote hidden}>I'm working on a project (currently using the >PIC16F84) in which I would like to be able to set the >servo (1-8) to a specific setting (1=Extreme >left/128=center/255=extreme right -- or whatever) >somewhere in the main loop, and have an interrupt take >care of actually maintaining that setting in the >background. I'm writing this is MPASM/GPASM for speed >& compatibility. > >I've looked at a number of servo control source, and I >haven't found anybody else doing it this way (it >appears everyone dedicates a 16f84 to this task). I >figure this would have to be table driven, but just >can't get this to work. The timing is something else >I'm having trouble getting just right. Both chips are >clock externally w/a 20mhz crystal. > >I can't believe I am the only person that has ever >wanted to do this, does anybody out there have any >examples I can look at that perhaps I've missed? > >I'm not new to programming (just new to PIC's), and >it's killing me that I can't figure this out. I've >been working it for days now... I can send the code I >have currently if anybody is interested.. > >-- Rob > > >__________________________________________________ >Do You Yahoo!? >Get personalized email addresses from Yahoo! Mail - only $35 >a year! http://personal.mail.yahoo.com/ > >-- >http://www.piclist.com hint: The PICList is archived three different >ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

>>My application uses 2 servo motors, here is what I do and it works well: >>in a loop: >>- Set high bits bits that act as signals for the servos: >>- Wait 1mSec >>- Set low the bits of the motors that needs to be on one side. >>- Wait 0.5mSec >>- Set low the bits of the motors that needs to be on the center. >>- Wait 0.5mSec >>- Set low the bits of the motors that needs to be on the other side. >>- Wait 18mSec. <<SNIP>> I didn't realise that you could drive 2 servos with a 16F84, so do you need to add say 2 hardware timers etc.? Could you kindly provide some sample code snippets and schematics for a newbie? TIA, David -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> >I didn't realise that you could drive 2 servos with a 16F84, so do you need >to add say 2 >hardware timers etc.? > >Could you kindly provide some sample code snippets and schematics for a >newbie? One timer. You update each servo in turn. You're out of servos when you're out of pins :) (And you can get clever about that) The servos normally get a pulse about every 20mS. So, you can output as many as 10 servos without them noticing, simply by taking turns. You can do it in paralell too, but that's a little harder. -- Dave's Engineering Page: http://www.dvanhorn.org Where's dave? http://www.findu.com/cgi-bin/find.cgi?kc6ete-9 -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

----- Original Message ----- From: David VanHorn <spam_OUTdvanhornTakeThisOuTCEDAR.NET> To: <.....PICLISTKILLspam@spam@MITVMA.MIT.EDU> Sent: Sunday, February 11, 2001 2:36 PM Subject: Re: [PIC]: PIC16F84/16F877 ISR SERVO CONTROL > > > >I didn't realise that you could drive 2 servos with a 16F84, so do you need > >to add say 2 > >hardware timers etc.? > > > >Could you kindly provide some sample code snippets and schematics for a > >newbie? > > One timer. > > You update each servo in turn. You're out of servos when you're out of pins :) > (And you can get clever about that) An easy way to get 8 servos out of 4 pins: 74xx138 decoder. Bob Ammerman RAm Systems (contract development of high performance, high function, low-level software) -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

74xx138 decoder. I also really like that chip I use two of them to map two data lines to 16. The cool thing is that after I select one of the 8 possible addresses, I can toggle the two pins (or make on an input and toggle the other) with out having to reload an entire shift register or whatever. This means that the data lines on the other 7 address sets are held steady so you can twiddle this over here, then leave it and twiddle that over there without that disturbing this or this disturbing that or... confused? Anyway, it seems like the bulk of what I do needs two pins to twiddle really fast at the same time. I2C, (SDA, SLC are fast) SPI (SCK, SIO (combined in and out) are fast and nCS is slow on another address or port pin ) External BUS type devices like memory or parallel ports need an 8 bit register on a bus and then the 138 provides EN and clock, etc... James Newton, PICList Admin #3 jamesnewtonKILLspampiclist.com 1-619-652-0593 phone http://www.piclist.com {Original Message removed}