Post by thread:[PIC]: UART Rx Problems

Hello All, I am having trouble with the UART of an 16F877. I am using a piece of software which simulates the PIC extremely well and does simulate the UART (Proteous). Every time I initialise the UART the Rx State changes to "Start Bit" straight away despite the input being configured as an input and its being driven high. The UART goes to start bit as soon as the instruction: bsf rcsta, cren is executed. Because the UART thinks it has recived a start bit, it gets a framing error which is impossible to clear for some reason. Mabe its because the UART status goes immediatly to start bit again after recieveing anything ???? Thanks for any help in advance -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

On Wed, 28 Nov 2001, Laurence Evans wrote: > Hello All, > > I am having trouble with the UART of an 16F877. > > I am using a piece of software which simulates the PIC extremely > well and does simulate the UART (Proteous). Every time I initialise > the UART the Rx State changes to "Start Bit" straight away > despite the input being configured as an input and its being driven > high. So are you having this problem with a real 16F877, or only on the simulator? I understand that the simulator may be really good, but it may still not be quite 100% perfect... also, make sure you're configuring both the Tx and Rx pins (RB6 & 7) as inputs. I doubt this would cause what you're seeing, though. > The UART goes to start bit as soon as the instruction: > > bsf rcsta, cren > > is executed. Because the UART thinks it has recived a start bit, it > gets a framing error which is impossible to clear for some reason. > Mabe its because the UART status goes immediatly to start bit > again after recieveing anything ???? I'm not sure what you mean by "UART goes to start bit"... do you mean the receiver thinks it's seeing a start bit? How do you know this? There's an RCIF bit to tell you when the*stop* bit has been received, but nothing to tell you the start bit is being received unless it's in your sim. Also, if you're driving the PI pin high, remember that the data is true at teh PIC, not inverted like you'll see it on the other side of the RS232 level shifter. A 1488/89 or MAX232-type driver/receiver will invert the data. So a logic high at the PIC pin *is* a start bit. > Thanks for any help in advance Hope this helps. If not you might want to post the UART init code you're using... Dale -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Hi Laurence, Go to http://redrival.com/mcgahee/ and download PICUART.ZIP, which is Fr. Tom McGahee's PIC UART tutorial. I think this is a known "gotcha" and there is a way to clear the framing error. It is addressed in his (well commented) example code. Sean At 12:48 PM 11/28/01 +0100, you wrote: {Quote hidden}>Hello All, > >I am having trouble with the UART of an 16F877. > >I am using a piece of software which simulates the PIC extremely >well and does simulate the UART (Proteous). Every time I initialise >the UART the Rx State changes to "Start Bit" straight away >despite the input being configured as an input and its being driven >high. > >The UART goes to start bit as soon as the instruction: > >bsf rcsta, cren > >is executed. Because the UART thinks it has recived a start bit, it >gets a framing error which is impossible to clear for some reason. >Mabe its because the UART status goes immediatly to start bit >again after recieveing anything ???? > >Thanks for any help in advance > >-- >http://www.piclist.com hint: PICList Posts must start with ONE topic: >[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads ---------------------------------------------------- Sign Up for NetZero Platinum Today Only $9.95 per month! http://my.netzero.net/s/signup?r=platinum&refcd=PT97 -- 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 Laurence, I don't have a solution to your problem, unfortunately, but maybe you can help me. I also got the Proteus Virtual Simulator + the '874 model. However, I can't get it to work, not even a simple setting of output pins. I'm using a simple piece of software that runs on the hardware OK, but no success with the VSM. Could you mail me a source (spam_OUTjan.diddenTakeThisOuTskintech.be) that you know runs on the VSM so I can see what I maybe do wrong? Thanks beforehand, Jan Didden By the way PIClisters, it's worthwhile to check out http://www.labcenter.co.uk, their VSM simulates pic systems interactively, i.e. you mouse-click a switch on the schematic and the pic reacts etc. Including interactive debugging etc. Very neat! There is a lite version for < 100 US$ {Original Message removed}

If you are driving the pin high, then you have a start bit and you'll get a framing error until you drive it low. If you clear the framing error, it will come right back unless you drive the pin low. If you need to drive the pin high for some reason, either turn off the rx interrupt, or turn off the uart rx. Remember that in 0-5v logic, rs-232 is not inverted, so a 1 is +5v. A start bit is +5v, stop bit is 0v. -Adam Laurence Evans wrote: {Quote hidden}>Hello All, > >I am having trouble with the UART of an 16F877. > >I am using a piece of software which simulates the PIC extremely >well and does simulate the UART (Proteous). Every time I initialise >the UART the Rx State changes to "Start Bit" straight away >despite the input being configured as an input and its being driven >high. > >The UART goes to start bit as soon as the instruction: > >bsf rcsta, cren > >is executed. Because the UART thinks it has recived a start bit, it >gets a framing error which is impossible to clear for some reason. >Mabe its because the UART status goes immediatly to start bit >again after recieveing anything ???? > >Thanks for any help in advance > >-- >http://www.piclist.com hint: PICList Posts must start with ONE topic: >[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads > > > > > -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email .....listservKILLspam@spam@mitvma.mit.edu with SET PICList DIGEST in the body

On Sat, 1 Dec 2001 09:53:07 -0500, you wrote: >If you are driving the pin high, then you have a start bit and you'll >get a framing error until you drive it low. If you clear the framing >error, it will come right back unless you drive the pin low. > >If you need to drive the pin high for some reason, either turn off the >rx interrupt, or turn off the uart rx. > >Remember that in 0-5v logic, rs-232 is not inverted, so a 1 is +5v. A >start bit is +5v, stop bit is 0v. > Are you sure about that? I think you have it backwards. All of the UART timing diagrams that I've seen in Microchip docs show the idle RS-232 input as a high on the Rx pin. (See PIC16F87X, DS30292B, page 101, figure 10-5) The start bit is detected when the RX pin goes low. The stop bit is shown as a high. Regards, Bob -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

You are right Bob, Every hardware UART I've seen in the last 20 years idles high, then goes low for a start bit (at the TTL or logic level). A constant low on the output is a "break" condition. Matt Pobursky Maximum Performance Systems On Sat, 1 Dec 2001 18:33:57 GMT, Bob Barr wrote: {Quote hidden}>On Sat, 1 Dec 2001 09:53:07 -0500, you wrote: > >>If you are driving the pin high, then you have a start bit and >>you'll >>get a framing error until you drive it low. If you clear the >>framing >>error, it will come right back unless you drive the pin low. >> >>If you need to drive the pin high for some reason, either turn >>off the >>rx interrupt, or turn off the uart rx. >> >>Remember that in 0-5v logic, rs-232 is not inverted, so a 1 is >>+5v. A >>start bit is +5v, stop bit is 0v. >> > >Are you sure about that? I think you have it backwards. > >All of the UART timing diagrams that I've seen in Microchip docs >show >the idle RS-232 input as a high on the Rx pin. (See PIC16F87X, >DS30292B, page 101, figure 10-5) The start bit is detected when >the RX >pin goes low. The stop bit is shown as a high. > >Regards, Bob > -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email .....listservKILLspam.....mitvma.mit.edu with SET PICList DIGEST in the body