They made an hcu04 unbuffered part specifically for oscillators. On Apr 20, 2013 10:57 AM, "Peter P." wrote: > Problems with 74HC(T)xxxx and CMOS4xxx IC 'linear' circuits. > > The following reproducible problems were found in several > popular 'analog' applications of CMOS gate based circuits. > The circuits are tried-and-true 'old' circuits from the > 1980s and 1970s, as published in datasheets and application > notes from various name silicon makers. > > 1 - inverter-with-feedback-resistor does not work or oscillates > uncontrollably. Occurs with 74HC(T)xxx circuits, any. Using any > feedback resistor over a simple inverter in a 74HC(T) circuit > causes the gate to oscillate at over 10 MHz. The resistor value > does not matter. The oscillation also occurs if an external > precise bias source is used to bias the input, without using a > feedback resistor. Source impedances tried ranged from 1 ohm to > 10Megohms. Consequences: > > - It is not possible to build low frequecy Xtal oscillators > using 74HC(T)xxxx gates. Example: 32767kHz osc on 74HC(T)4060 > will not work, it oscillates at several MHz. Manipulating the > usual Pierce oscillator part values around the xtal does not > help. This does not appear anywhere in the datasheet. > > 2 - CMOS oscillators built as above with gate-and-feedback > resistor stop working when the supply voltage is taken up > above 5V. Example: CD4060 with 32768kHz Xtal using datasheet > Pierce oscillator values (2.2kohms, 2x33pF, 2M2 feedback), > works fine at 5V but stops working at about 7V supply and is > impossible to start at 12V. My assumption is, that there is > some 'new' 'feature' caused by die shrinks which increases > leakage at the input and/or lowers output impedance somehow, > or that gain decreases with voltage somehow in the 'deadband' > dc bias region. > > In all cases I tried several Xtals (3 makes), and several ICs > (2 or 3 makes each). All ICs were new (datecode >2007). I did > not try old old parts (CD4060 etc) beucause I know they work > fine. > > Any comments would be welcome here. I am a little stumped by > these new features. I tried the above several times (incredu- > lously, after having used cmos parts since the early 1980s > with no problems). > > What IS going on, in your opinion? > > Comments, etc., are very welcome. Tia, > > -- Peter > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .