Post by thread:[PIC] 18F2550 Analog I/O Limits

Hello Everyone, I need to make a design decision where I need to drive ; 1- a ppm laser diode module (integrated apm - TE cooling) 2- a PMT (photo multiplier tube) 3- an APD (avalanche photo-diode) 4- a unipolar/bipolar stepper motor 5- An analog input of 0-10VDC Hands currently on a PIC 18F2550 conducting usb data transfer. It seems that 18F2550 is capable of handling 5 analog inputs simultaneously where I have question marks about the overall performance and response of this MCU. I want the system to be compact and smart where it seems that all necessary algorithms can be handled by only one amount of 18f2550. Since I didn't push the limits (bandwidth - performance) of this unit I don't know if it's capable of doing the job as expected. Do you think this mcu can simultaneously read 5 analog inputs and send the data through USB without sacrificing any performance or precision ? I don't want (who wants right) an obstacle in development after I start the process which will change the deadline and thus roadmap of the project. I am sure more experienced PIC users will quickly come up with an answer, also any guidance is appreciated. Have a nice da

On 06/06/2011 20:59, Yigit Turgut wrote: > Do you think this mcu can simultaneously > read 5 analog inputs and send the data through USB without sacrificing > any performance or precision ? If all you want it to do is exactly the above, (i.e. take five analogue voltage readings) then I would say yes, it could do that no problem. It would help to know in a little more detail what exactly the PIC will be doing though. Will there be any processing of the analogue information? (what kind? simple maths or more involved like e.g. FFT) You mentioned driving the various components too, will the PIC be involved in this also? Will there be any other things for the PIC to do? (display, user I/O, etc) Depending on your answer, it may be worth looking at a PIC24/32/dsPIC instead.

there is no pic to read 5 analog at the same time. they all multiplexed . I use pic24 with ccs a lot whey are great part Andre ________________________________ From: Yigit Turgut <spam_OUTy.turgutTakeThisOuTgmail.com> To: Microcontroller discussion list - Public. <.....piclistKILLspam@spam@mit.edu> Sent: Mon, June 6, 2011 12:59:05 PM Subject: [PIC] 18F2550 Analog I/O Limits Hello Everyone, I need to make a design decision where I need to drive ; 1- a ppm laser diode module (integrated apm - TE cooling) 2- a PMT (photo multiplier tube) 3- an APD (avalanche photo-diode) 4- a unipolar/bipolar stepper motor 5- An analog input of 0-10VDC Hands currently on a PIC 18F2550 conducting usb data transfer. It seems that 18F2550 is capable of handling 5 analog inputs simultaneously where I have question marks about the overall performance and response of this MCU. I want the system to be compact and smart where it seems that all necessary algorithms can be handled by only one amount of 18f2550. Since I didn't push the limits (bandwidth - performance) of this unit I don't know if it's capable of doing the job as expected. Do you think this mcu can simultaneously read 5 analog inputs and send the data through USB without sacrificing any performance or precision ? I don't want (who wants right) an obstacle in development after I start the process which will change the deadline and thus roadmap of the project. I am sure more experienced PIC users will quickly come up with an answer, also any guidance is appreciated. Have a nice da

On 06/06/2011 20:59, Yigit Turgut wrote: > Do you think this mcu can simultaneously... Simple answer is *no*. It always reads them one at a time. It will *never* read them at exactly the same time. Now, what *exactly* does "simultaneously" mean for you ?? Is there a limit in a specific number of milli or micro seconds that still is regarded as "simultaneously" ? You could add external sample/hold curcuits to "lock" the inputs at a common timestamp and than read them from the PIC. Or use another PIC (PIC24/30/33) that has multiple S/H curcuits built-in. Jan-Erik

On 06/06/2011 20:59, Yigit Turgut wrote: > It > seems that 18F2550 is capable of handling 5 analog inputs > simultaneously Sorry I missed this part - do you mean absolutely simultaneously? Or would very close together do? As Andre pointed out, the analog inputs are multiplexed together, so you can't read two or more channels exactly at the same time. You can set the converter to read them all sequentially one after the other though. There is a setting on some PICs (e.g. PIC32 and probably many newer PICs) in the ADC register to do this, but on the 18F2550 I *think* you would have to do it in software.

On Mon, Jun 6, 2011 at 4:52 PM, Jan-Erik Soderholm <jan-erik.soderholmKILLspamtelia.com> wrote: > Â On 06/06/2011 20:59, Yigit Turgut wrote: > >> Do you think this mcu can simultaneously... > > Simple answer is *no*. > It always reads them one at a time. > It will *never* read them at exactly the same time. > > Now, what *exactly* does "simultaneously" mean for you ?? > Is there a limit in a specific number of milli or micro > seconds that still is regarded as "simultaneously" ? > > You could add external sample/hold curcuits to "lock" > the inputs at a common timestamp and than read them > from the PIC. Or use another PIC (PIC24/30/33) that > has multiple S/H curcuits built-in. > Even the biggest-baddest dsPIC33 can't S/H 5 analog inputs at once. I think the requirement of "at once" is likely to just as easily mean: "within a few tens of microseconds of each other" Yigit, what is the required sample rate? -- Martin K.

I used pic18f4550 same as 2550 and completed a project using 6 analog joysticks plus 16 buttons multiplexed , rs422 and usb. it worked fine. you can use 2550 to get your job done. pic24/32 are faster parts better but if you never worked with them beginning you may have headache until make them work specially USB hid. AA ________________________________ From: Yigit Turgut <.....y.turgutKILLspam.....gmail.com> To: Microcontroller discussion list - Public. <EraseMEpiclistspam_OUTTakeThisOuTmit.edu> Sent: Mon, June 6, 2011 12:59:05 PM Subject: [PIC] 18F2550 Analog I/O Limits Hello Everyone, I need to make a design decision where I need to drive ; 1- a ppm laser diode module (integrated apm - TE cooling) 2- a PMT (photo multiplier tube) 3- an APD (avalanche photo-diode) 4- a unipolar/bipolar stepper motor 5- An analog input of 0-10VDC Hands currently on a PIC 18F2550 conducting usb data transfer. It seems that 18F2550 is capable of handling 5 analog inputs simultaneously where I have question marks about the overall performance and response of this MCU. I want the system to be compact and smart where it seems that all necessary algorithms can be handled by only one amount of 18f2550. Since I didn't push the limits (bandwidth - performance) of this unit I don't know if it's capable of doing the job as expected. Do you think this mcu can simultaneously read 5 analog inputs and send the data through USB without sacrificing any performance or precision ? I don't want (who wants right) an obstacle in development after I start the process which will change the deadline and thus roadmap of the project. I am sure more experienced PIC users will quickly come up with an answer, also any guidance is appreciated. Have a nice da

On 06/06/2011 21:52, Jan-Erik Soderholm wrote: > Or use another PIC (PIC24/30/33) that > has multiple S/H curcuits built-in. I was just thinking about this - I know there are dsPICs with multiple SHA, but I wasn't aware of any with more than 4. I haven't looked for a while though. A separate SHA IC would be an option if sampling at the exact same time is needed.

Sampling rate is around 10KHz and is expected to increase while ongoing development. This will occur where precision will matter. How the multiplexing is done inside ? If it's a constant amount then I can correlate the results at PC software side otherwise I guess I will need to increase the budget and upgrade to a better platform. On Tue, Jun 7, 2011 at 12:03 AM, M.L. <mspam_OUTlkeng.net> wrote: {Quote hidden}> On Mon, Jun 6, 2011 at 4:52 PM, Jan-Erik Soderholm > <@spam@jan-erik.soderholmKILLspamtelia.com> wrote: >> On 06/06/2011 20:59, Yigit Turgut wrote: >> >>> Do you think this mcu can simultaneously... >> >> Simple answer is *no*. >> It always reads them one at a time. >> It will *never* read them at exactly the same time. >> >> Now, what *exactly* does "simultaneously" mean for you ?? >> Is there a limit in a specific number of milli or micro >> seconds that still is regarded as "simultaneously" ? >> >> You could add external sample/hold curcuits to "lock" >> the inputs at a common timestamp and than read them >> from the PIC. Or use another PIC (PIC24/30/33) that >> has multiple S/H curcuits built-in. >> > > Even the biggest-baddest dsPIC33 can't S/H 5 analog inputs at once. > > I think the requirement of "at once" is likely to just as easily mean: > "within a few tens of microseconds of each other" > > Yigit, what is the required sample rate? > > -- > Martin K. > >

> Sampling rate is around 10KHz and is expected to increase while > ongoing development. This will occur where precision will matter. How > the multiplexing is done inside ? It is done the same way the 16F pics have done it for ages, with an analogue multiplexor. The appropriate channel is selected by loading a binary value into a register. -- Scanned by iCritical.

Yigit Turgut wrote: > Sampling rate is around 10KHz and is expected to increase while > ongoing development. This will occur where precision will matter. How > the multiplexing is done inside ? If it's a constant amount then I > can correlate the results at PC software side otherwise I guess I > will need to increase the budget and upgrade to a better platform. Look at the block diagram of the ADC inputs in the datasheet to see how the inputs are multiplexed inside. If precision matters enough, you can trigger the sampling from a timer interrupt, making sure you leave enough time for a full conversion between interrupts. This way you have always the exact same time between conversions. Gerhar