Post by thread:I/O Impedance

In message <spam_OUT0043C076.4149TakeThisOuTautodesk.com>, ERIC SCHLAEPFER <.....eric.schlaepferKILLspam@spam@AUTODESK.COM> writes >Hello, > >Does anyone know the input and output impedance of each I/O line in PORTB >of a 16F84? I can't find it in the data sheet. I don't think that impedance is very meaningful in this context, which is why it isn't quoted. The AC output impedance is probably highly frequency- and load-dependent, unlike a linear amplifier. I've checked through some of the different manufacturers' data I've got, and can't find any who specify it. You could probably model it for a given set of conditions, if you knew the characteristics of the input/output devices. Inmos used SPICE simulation to characterise the transistors used in the transputer, for instance. I've just looked at the data for a VHF mixer device. This is an analogue chip, and the mixer input impedance varies quite a lot over the design frequency range. Digital devices would be much messier. Leon -- Leon Heller: leonKILLspamlfheller.demon.co.uk http://www.lfheller.demon.co.uk Amateur Radio Callsign G1HSM Tel: +44 (0) 118 947 1424 See http://www.lfheller.demon.co.uk/dds.htm for details of my AD9850 DDS system. See " "/diy_dsp.htm for a simple DIY DSP ADSP-2104 system.

On Thu, 26 Mar 1998 11:26:14 -0800 ERIC SCHLAEPFER <.....eric.schlaepferKILLspam.....AUTODESK.COM> writes: >Hello, > >Does anyone know the input and output impedance of each I/O line in >PORTB >of a 16F84? I can't find it in the data sheet. A DC "input impedance" can be inferred from the specified DC leakage current. The specification is about 1uA. Depending on the chip conditions, the current may leak from any voltage between Vss and Vdd. Assuming the worst case, such as input at Vdd and all leakage current to Vss, with 5V Vdd an equivalent input circuit would be a resistance of 5V/1uA = 5 M ohm. So it is somewhat useful to declare that the input impedance (for input voltages between Vss and Vdd) is 5M ohm or more. A small-signal output impedance can be derived from the slope of the Vout versus Iout curves in the data sheet. The curves vary widely depending on Vdd, Ta, and Vout. When the output is driving "high", it has a lot more impedance than when it is driving "low". Consider a worst-case of 5V, Ta=85C. The high-level output voltage drops 1V (from 5V to 4V) with approximately 3 mA of output current. So the output impedance is about 1V/3 mA = 330 ohms. This is very much a worst-case. Under most conditions it will be much less. Also there is a capacitance of about 5 pF per pin that should be factored into the impedance for high-frequency signals. _____________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com Or call Juno at (800) 654-JUNO [654-5866]