Post by thread:[PIC]: Supply voltage and USART

I've been getting some strange problems with a PIC16F877 relating to its USART. I'm running in synchronous mode, and That worked fine on the breadboard. Like a charm, even. But, then when I ported it over to PCB, it started misbehaving. I'm confused as to what the problem actually is. I'm running a bit-banged interface with a PS/2 device, and it appears to work correctly. and has worked correctly on a breadboard. I'm outputting synchronous serial data via the USART. I use code that ouputs a byte, waits for completion, then outputs another. 4 bytes in total. While the synchronous data is being transmitted, the PS/2 is locked out. The problem is that when I attach the receiver for the synchronous data, the stream begins to transmit incessantly. It is unstable up to roughly 5.1V. Therefore, I'm led to wonder if the voltage applied affects the USART at all. The peripheral that attaches to the PIC USART draws almost 500mA *from the power rails*. I have tried attaching it to a seperate power supply, and I get the same result, but if I increase the voltage to the PIC slightly, it seems to stabilize a little, however, the USART seems to affect the bitbanged interface, and is causing it to abort transmissions part way through. I've no idea what's happening. Anyone else run into something like this before? (believe me, I'm rechecking all my communication code, even though it worked before) --Brendan -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> I've been getting some strange problems with a PIC16F877 relating to its > USART. I'm running in synchronous mode, and That worked fine on the > breadboard. Like a charm, even. But, then when I ported it over to PCB, it > started misbehaving. I'm confused as to what the problem actually is. Well, it looks like it was something that I could repair in code. But I still want to know why the *voltage* was affecting the stability despite the code problem, and why the code had worked in another situation. Probably capacitance in the breadboard connectors, eh? --Brendan -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> The problem is that when I attach the receiver for the synchronous data, the > stream begins to transmit incessantly. It is unstable up to roughly 5.1V. > Therefore, I'm led to wonder if the voltage applied affects the USART > at all. The peripheral that attaches to the PIC USART draws > almost 500mA *from the power rails*. I have tried attaching it to a > seperate power supply, and I get the same result, but if I increase the > voltage to the PIC slightly, it seems to stabilize a little, however, the > USART seems to affect the bitbanged interface, and is causing it to abort > transmissions part way through. Are there large glitches on the line perhaps causing the protection diodes to kick in? Who is driving the data and clock lines when? It sounds like there is some kind of collision. What do you mean by "unstable up to roughly 5.1V"? ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

Hi there, sound like you possibly have PSU noise issues. try an "under then chip" (thin film, looks like a chip itself) cap and or a nice 1u Tant cap on its supply. I has a similar problem with my 18F's and that seemed to solve it. The problem arose as the uart tried to drive a low impedance peripheral which made the power line dip slightly.... it was enough to upset the poor thing. On Saturday 31 August 2002 01:43 pm, you wrote: {Quote hidden}> > The problem is that when I attach the receiver for the synchronous data, > > the > > > stream begins to transmit incessantly. It is unstable up to roughly > > 5.1V. Therefore, I'm led to wonder if the voltage applied affects the > > USART at all. The peripheral that attaches to the PIC USART draws > > almost 500mA *from the power rails*. I have tried attaching it to a > > seperate power supply, and I get the same result, but if I increase the > > voltage to the PIC slightly, it seems to stabilize a little, however, the > > USART seems to affect the bitbanged interface, and is causing it to abort > > transmissions part way through. > > Are there large glitches on the line perhaps causing the protection diodes > to kick in? Who is driving the data and clock lines when? It sounds like > there is some kind of collision. What do you mean by "unstable up to > roughly 5.1V"? > > > ***************************************************************** > Embed Inc, embedded system specialists in Littleton Massachusetts > (978) 742-9014, http://www.embedinc.com -- Thank-you for your time. John Ward -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spam_OUTlistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body