Wagner and Brent--Thanks for the quick reply-- Not familiar with the Atmel, and only could find a summary data sheet on their web page---Hence the confusion on Pins versus ports. I am interested in physical pins on the part-- #6 and #7 whatever they are called. The Atmel and Max218 are being run at 4.5 volts nominal, and the RFM transceiver is running at 3.0 volts--so there is a level shift needed between the atmel and the RFM chip. Pin Six is being used for input from the collector (connected only to the atmel) of a transistor getting a 3 volt digital signal from the RFM transceiver (RS232 type stuff, but logic levels) to the base, with the Emitter tied to ground (2N2222). I am assuming for this to work, there must be a pull up to 4.5 volts somewhere to work with the transistor--since it is not on the PCB, I would guess it is in the Atmel. I am trying to figure out the value of the internal pull up resistor, so I can duplicate the circuit externally for a test. It is also providing an inverting function to correct the transceivers logic levels to match the atmel--polarity as well as voltage. Pin Seven is being used for an output to drive the transmit data function of the RFM transmitter, and only has a 10K pull up to 3.0 volts on it. Again, I am guessing that the Atmel has an open collector sort of driver on that pin, and is being used in a non-inverting level-shifting function. Hope this clarifies what I am looking to have confirmed. Kelly At 02:16 PM 9/24/99 +1200, you wrote: >> Hi you Atmel types: >> >> Have an application we have to convert to a PIC in the very near future, >> and am trying to figure out how the circuit we have been given is working. >> >> Two Questions: >> Port 3, Pin 6, setup for input--does it have an internal pull up resistor, >> and if so, what is the value (approximately)? >> >> Port #, Pin 7, setup for output--is this an open collector/drain type >> driver? Pin has a pull up to 3.0 volts for level shifting from the 4.5V >> rail--and this is the only way I can figure that it would work. >> >> Thanks, >> Kelly >> >> >> William K. Borsum, P.E. -- OEM Dataloggers and Instrumentation Systems >> & San Diego, California, USA >> > >P3.6 does not physically exist as a port pin, it is used for reading >the output of the analog comparator. Treat it like a digital input, no >need to worry about pull ups or anything. > >P#.7? Looking at my keyboard I think you mean P3.7 This is a >standard I/O pin with weak internal pull up. As a rule assume this >pullup is approx 30k (calculated from DC specs) but remember that >it is non-linear. Sounds like in your circuit the external pullup >resistor (<30k?) overrides this and gives you a rough 3V output. > >The only two open drain pins are P1.0 and P1.1 > >Changing to a PIC? Good - do it - but only if it offers you some >advantages for your application. You might look at the AVR >AT90S2313. Drop in replacement for the AT89C2051 except for >opposite reset polarity. > >Brent Brown >Electronic Design Solutions >16 English Street >Hamilton, New Zealand >Ph/fax: +64 7 849 0069 >Mobile: 025 334 069 >eMail: brent.brown@clear.net.nz > > William K. Borsum, P.E. -- OEM Dataloggers and Instrumentation Systems & San Diego, California, USA