Post by thread:[PIC]: 16F874 USART for DMX-512

Hi, I'm working on DMX-512 receiving code with the PIC16F874 using the built in USART. DMX uses a break to start a new packet. It is supposed to be 88uS (2 frame times @250Kbps), and the recommended practice says that receivers should detect on slightly less, like 70uS. So far I'm using the framing error flag to detect a break by making sure that all the data bits are low. I've been getting a bit of instability with this method. Is there a better way? Also, the PIC16F874 datasheet is confusing when it talks about how to read the error flag bits in the receiver. It says that the FERR flag gets buffered in the FIFO, but the block diagram wouldn't suggest that. Then it says to read the error flags before requesting a byte from the FIFO. If FERR flags are buffered in the FIFO wouldn't this mean that you'd be reading the wrong data unless you check the FERR bit after you get data from the FIFO? Can someone please explain this? Thanks, Andrew -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> So far I'm using the framing error flag to detect a break by > making sure that all the data bits are low. I've been getting a bit > of instability with this method. Is there a better way? I have absolutely no idea, but did you check http://www.qsl.net/n5tle/ ? I helped someone to get the 16f876 receiver working and (once the hardware was OK) it worked flawlessly. Wouter van Ooijen -- Van Ooijen Technische Informatica: http://www.voti.nl Jal compiler, Wisp programmer, WLoader bootloader, PICs kopen -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

On Thu, 21 Feb 2002 14:17:17 -0500 Andrew Kilpatrick <spam_OUTandrewTakeThisOuTANDREWKILPATRICK.ORG> writes: {Quote hidden}> Hi, > > I'm working on DMX-512 receiving code with the PIC16F874 using > the built in USART. DMX uses a break to start a new packet. It > is supposed to be 88uS (2 frame times @250Kbps), and the recommended > practice says that receivers should detect on slightly less, like > 70uS. > > So far I'm using the framing error flag to detect a break by > making sure that all the data bits are low. I've been getting a bit > of instability with this method. Is there a better way? > > Also, the PIC16F874 datasheet is confusing when it talks about how > to read the error flag bits in the receiver. It says that the FERR > flag gets buffered in the FIFO, but the block diagram wouldn't > suggest > that. Then it says to read the error flags before requesting a byte > from the FIFO. If FERR flags are buffered in the FIFO wouldn't this > mean that you'd be reading the wrong data unless you check the FERR > bit after you get data from the FIFO? Can someone please explain > this? > I've done a fair amount of DMX transmission and reception using PICs (mostly 16c74 and 18c452). I detect a break using the FE flag, then "eat the byte" out of the usart data register, then go back to waiting for the next incoming byte. After the FE, we assume the incoming byte will be the start code byte (which should be zero). As each byte is brought in, FE is checked. If an FE occurs, the state machine is reset to the point where we are waiting for the start code byte. I don't check the contents of the byte when there was an FE. I just throw it out. Seems to work (many thousand units now out there). Harold FCC Rules Online at http://hallikainen.com/FccRules Lighting control for theatre and television at http://www.dovesystems.com ________________________________________________________________ GET INTERNET ACCESS FROM JUNO! Juno offers FREE or PREMIUM Internet access for less! Join Juno today! For your FREE software, visit: dl.http://www.juno.com/get/web/. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Hi Harold, Thanks for the response. I'm still unclear as to when is the time to check the error flags... before reading the byte from the data register (if there is one) or after. I've seen code that does it both ways, but the datasheet for the PIC suggests that you should read the error flags first. Can you clear this up for me? Thanks. Andrew {Quote hidden}> > I've done a fair amount of DMX transmission and reception using PICs > (mostly 16c74 and 18c452). I detect a break using the FE flag, then "eat > the byte" out of the usart data register, then go back to waiting for the > next incoming byte. After the FE, we assume the incoming byte will be the > start code byte (which should be zero). As each byte is brought in, FE is > checked. If an FE occurs, the state machine is reset to the point where > we are waiting for the start code byte. I don't check the contents of the > byte when there was an FE. I just throw it out. Seems to work (many > thousand units now out there). > > Harold > > > > FCC Rules Online at http://hallikainen.com/FccRules > Lighting control for theatre and television at http://www.dovesystems.com > > ________________________________________________________________ > GET INTERNET ACCESS FROM JUNO! > Juno offers FREE or PREMIUM Internet access for less! > Join Juno today! For your FREE software, visit: > dl.http://www.juno.com/get/web/. > > -- > 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 hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

On Thu, 21 Feb 2002 16:06:43 -0500 Andrew Kilpatrick <.....andrewKILLspam@spam@ANDREWKILPATRICK.ORG> writes: > Hi Harold, > > Thanks for the response. I'm still unclear as to when is the time to > check the error flags... before reading the byte from the data > register > (if there is one) or after. I've seen code that does it both ways, > but > the datasheet for the PIC suggests that you should read the error > flags > first. Can you clear this up for me? Thanks. > I read them first, then read the data byte. If I had a framing error, I reset the state machine and throw out the received byte. Harold FCC Rules Online at http://hallikainen.com/FccRules Lighting control for theatre and television at http://www.dovesystems.com ________________________________________________________________ GET INTERNET ACCESS FROM JUNO! Juno offers FREE or PREMIUM Internet access for less! Join Juno today! For your FREE software, visit: dl.http://www.juno.com/get/web/. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> Also, the PIC16F874 datasheet is confusing when it talks about how > to read the error flag bits in the receiver. Don't blame this on the manual. Actually it's quite clear. > It says that the FERR > flag gets buffered in the FIFO, but the block diagram wouldn't suggest > that. The block diagram is just that, not a schematic. > Then it says to read the error flags before requesting a byte > from the FIFO. If FERR flags are buffered in the FIFO wouldn't this > mean that you'd be reading the wrong data unless you check the FERR > bit after you get data from the FIFO? Can someone please explain this? In section 10.2.2 "USART Asynchronous Receiver" on page 101 of DS3029C near the bottom of the second colum of text it says "Reading RCREG will load bit RX9D and FERR with new values, therefore, it is essential for the user to read the RCSTA register *before* reading the RCREG register in order not to lose the old FERR and RX9D information". I don't think I can explain it any better than that. ******************************************************************** Olin Lathrop, embedded systems consultant in Littleton Massachusetts (978) 742-9014, olinKILLspamembedinc.com, http://www.embedinc.com -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Hello Harold, > I've done a fair amount of DMX transmission and reception using PICs > (mostly 16c74 and 18c452). I detect a break using the FE flag, then "eat > the byte" out of the usart data register, then go back to waiting for the have you tried enabling 9 bit reception and using the RCSTA.RX9D bit to check for a break? I'm actually doing this and it's workin ok. But I've been thinking lately that checking for the FE would be better because the USART will be receiving only 8 bits and not 9 bits, so if the device sending DMX is a bit fast and sends only 1 stop bit it will still work. Best regards, Brusque ----------------------------------------------------------------- Edson Brusque C.I.Tronics Lighting Designers Ltda Researcher and Developer Blumenau - SC - Brazil Say NO to HTML mail http://www.citronics.com.br ----------------------------------------------------------------- -- 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

> Thanks for the response. I'm still unclear as to when is the time to > check the error flags... before reading the byte from the data register > (if there is one) or after. I've seen code that does it both ways, but > the datasheet for the PIC suggests that you should read the error flags > first. Can you clear this up for me? Thanks. As the data sheet says, you read the FERR flag first. Reading RCREG advances the FIFO, which includes the FERR flag. Therefore, FERR is always **for the next byte to be read** from RCREG. ******************************************************************** Olin Lathrop, embedded systems consultant in Littleton Massachusetts (978) 742-9014, .....olinKILLspam.....embedinc.com, 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

On Fri, 22 Feb 2002 09:29:54 -0300 Edson Brusque <EraseMEebrusquespam_OUTTakeThisOuTTERRA.COM.BR> writes: > > have you tried enabling 9 bit reception and using the RCSTA.RX9D > bit to > check for a break? I'm actually doing this and it's workin ok. > > But I've been thinking lately that checking for the FE would be > better > because the USART will be receiving only 8 bits and not 9 bits, so > if the > device sending DMX is a bit fast and sends only 1 stop bit it will > still > work. > But the standard calls for 2 stop bits, doesn't it? Harold FCC Rules Online at http://hallikainen.com/FccRules Lighting control for theatre and television at http://www.dovesystems.com ________________________________________________________________ GET INTERNET ACCESS FROM JUNO! Juno offers FREE or PREMIUM Internet access for less! Join Juno today! For your FREE software, visit: dl.http://www.juno.com/get/web/. -- 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

Hello Harold, > But the standard calls for 2 stop bits, doesn't it? yes, you're right. But... who knows. Better be safe. Best regards, Brusque ----------------------------------------------------------------- Edson Brusque C.I.Tronics Lighting Designers Ltda Researcher and Developer Blumenau - SC - Brazil Say NO to HTML mail http://www.citronics.com.br ----------------------------------------------------------------- -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.