Post by thread:Open-drain output

Hi Sami (Sami Khawam), in <34A81FF3.65E5KILLspamunet.univie.ac.at> on Dec 29 you wrote: > How can I detect, in an open-drain line, > a High signal from a device while the PIC > is outputing a Low signal? > Can a pin be read when the TRIS is set to > output? I don't know why you would want to do this? Insert a resistor serially into the line. Choose its value with the pullup value in mind. Make sure the pullup is on the remote side, not the PIC side. When the PIC outputs low and the remote does too, you will find 0V on the remote side. If the remote outputs 1, you will find several hundred mVs, depending on the shunt resistor value. O | pullup | _____ | PIC --------|_____|-----*----- remote ^ | measurement point

Gday Sami, Use an open drain pin on the pic,(eg RA4 on a 16c7x) and set it to be an output, then use it as normal, but when you read it you get the real value of the pin not what you are outputing. Dont Forget The Pull Up Resistor. regards Lee McLaren > Hi Sami (Sami Khawam), in <.....34A81FF3.65E5KILLspam.....unet.univie.ac.at> on Dec 29 you wrote: > > > How can I detect, in an open-drain line, > > a High signal from a device while the PIC > > is outputing a Low signal? > > Can a pin be read when the TRIS is set to > > output?

On Mon, 29 Dec 1997 23:10:59 +0100 Sami Khawam <EraseMEa9501901spam_OUTTakeThisOuTUNET.UNIVIE.AC.AT> writes: >Hello, >How can I detect, in an open-drain line, >a High signal from a device while the PIC >is outputing a Low signal? This is not a conventional application. Usually every device except one outputs a high signal, which is essentially an open circuit. The device which is currently allowed to send may output a low signal, which all devices on the line interpret as a low. The bus acts as a wired AND gate: the line reads high only if *all* connected devices are outputting high. So the PIC is to detect a Low signal from a device while the PIC is sending a High signal. If you really need to know if all *other* devices are outputting high except one, a possible method is to sense current flow from the bus to the OC pin with an external discrete transistor: +5V----| | Rp Rp *-------PIC Sense In | |/c Bus---*---| NPN | |\e |-R1--*------PIC OC In/Out R1 provides a path to guarantee that OC In reads low when the bus is low. It should be about the same resistance as Rp. If some other device is outputting a low (holding the bus at ground potential) the transistor will never turn on. Sense In will read high. If no other device is outputting a low except the PIC, the transistor turns on, and Sense In will read low. Sense In will read high when the PIC is outputting high in either case. While the PIC is outputting high, the OC In/Out pin can be read to determine the level on the bus. This is a special technique. Nearly all OC busses run with a protocol that avoids the need to do this. This circuit degrades the 'low' voltage the PIC can send to the bus by the Vbe of the transistor, which may cause problems. >Can a pin be read when the TRIS is set to >output? Yes. The value read is always determined by the voltage present at the pin, regardless of whether the pin is set as input or output.

Hello Mike. Wish you Happy New Year ! :-) {Quote hidden}> If you really need to know if all *other* devices are outputting high > except one, a possible method is to sense current flow from the bus to > the OC pin with an external discrete transistor: > > +5V----| > | Rp > Rp *-------PIC Sense In > | |/c > Bus---*---| NPN > | |\e > |-R1--*------PIC OC In/Out > > R1 provides a path to guarantee that OC In reads low when the bus is low. > It should be about the same resistance as Rp. > > If some other device is outputting a low (holding the bus at ground > potential) the transistor will never turn on. Sense In will read high. > If no other device is outputting a low except the PIC, the transistor > turns on, and Sense In will read low. Sense In will read high when the > PIC is outputting high in either case. While the PIC is outputting high, > the OC In/Out pin can be read to determine the level on the bus. There are also another idea how to make open-drain with only one PIC pin. Let us look at diagram below: 1. PIC pin is set as input 1a. Bus=HIGH => PIC pin=HIGH too (transistor in active mode) 1b. Bus=LOW => PIC pin=LOW too (transistor in saturation) 2. PIC pin is set as output 2a. PIC pin=HIGH => Bus=free (transistor is off) 2b. PIC pin=LOW => Bus=LOW (transistor in saturation) Bus | | |/c +5v--R1--| NPN |\e | R2 | PIC in/out pin R1 & R2 resistance resulting from Bus specification and type of transistor and may vary from 5KOhm to 500KOhm . WBR Dmitry.

On Thu, 1 Jan 1998 01:31:33 +0300 Dmitry Kiryashov <zewsspam_OUTAHA.RU> writes: [I wrote] >> While the PIC is outputting high, >> the OC In/Out pin can be read to determine the level on the bus. I should clarify that in order to 'output high' to an open collector bus, the PIC pin should be set to input mode (unless it is the RA4 pin, in which case it is OK to leave it output and set it high, since it has no pull-up output transistor). The pull-up resistor on the bus then raises the bus voltage to 'high' level. So OC In/Out is either set as output and low (to output low) or to input (to output high or input). It should not be set to output and high as this would supply extra current to devices on the bus (through R1 in this case, or directly if the sensing circuit isn't used). {Quote hidden}>There are also another idea how to make open-drain with only one PIC >pin. >Let us look at diagram below: > >1. PIC pin is set as input >1a. Bus=HIGH => PIC pin=HIGH too (transistor in active mode) >1b. Bus=LOW => PIC pin=LOW too (transistor in saturation) > >2. PIC pin is set as output >2a. PIC pin=HIGH => Bus=free (transistor is off) >2b. PIC pin=LOW => Bus=LOW (transistor in saturation) > > Bus > | > | > |/c > +5v--R1--| NPN > |\e > | > R2 > | > PIC in/out pin > This scheme does not solve the original poster's problem(?) of being able to tell if some other device is trying to pull the bus low at the same time the PIC is. Since the PIC pin must be forced low to turn the transistor on, it is not possible to read any meaningful value back in this condition. It is a useful circuit, however, to isolate the PIC from the bus while the PIC power is removed. The +5V to R1 would come from the same source that powers the PIC. When this voltage goes to 0, the transistor stays off all the time and no current flows from the bus to the PIC. Usually a FET is used as it has less voltage drop when the PIC is trying to pull the bus low. > >R1 & R2 resistance resulting from Bus specification and >type of transistor and may vary from 5KOhm to 500KOhm . I think in most cases R2=0 would be best. The value of R1 is not critical at all, it needs to be enough to bias the transistor on fully while the PIC is draining its maximum current from the bus. Using a FET, R1 could be omitted as well.

Hello Mike. {Quote hidden}> >1. PIC pin is set as input > >1a. Bus=HIGH => PIC pin=HIGH too (transistor in active mode) > >1b. Bus=LOW => PIC pin=LOW too (transistor in saturation) > > > >2. PIC pin is set as output > >2a. PIC pin=HIGH => Bus=free (transistor is off) > >2b. PIC pin=LOW => Bus=LOW (transistor in saturation) > > > > Bus > > | > > | > > |/c > > +5v--R1--| NPN > > |\e > > | > > R2 > > | > > PIC in/out pin > > > This scheme does not solve the original poster's problem(?) of being able > to tell if some other device is trying to pull the bus low at the same > time the PIC is. Since the PIC pin must be forced low to turn the > transistor on, it is not possible to read any meaningful value back in > this condition. The purpose of this circuit is to interface high voltage bus with "low" voltage PIC with minimal additional components. In my case i drive with this kind of scheme the 12v buses. Protocol features (detecting collision) usually used to detect bus status. > It is a useful circuit, however, to isolate the PIC from the bus while > the PIC power is removed. The +5V to R1 would come from the same source > that powers the PIC. When this voltage goes to 0, the transistor stays > off all the time and no current flows from the bus to the PIC. Usually a > FET is used as it has less voltage drop when the PIC is trying to pull > the bus low. I have few practic of FET using - what kind of FET you think i should apply in this circuit ? But i don't sure about FET using because in this case the bipolar go to saturation when low apply to collector or emitter. > >R1 & R2 resistance resulting from Bus specification and > >type of transistor and may vary from 5KOhm to 500KOhm . > I think in most cases R2=0 would be best. The value of R1 is not > critical at all, it needs to be enough to bias the transistor on fully > while the PIC is draining its maximum current from the bus. Using a FET, > R1 could be omitted as well. R2 have another aim - to prevent PIC damage when transistor is breake out due to any reason while bus voltage more than 6 volts. WBR Dmitry.