Russell McMahon wrote: > * Worst case character is arguably $AA as it contains a transition at > every possible bit edge. Not really. Remember, bytes are conventionally sent LSB first, so $AA does not have a transition at the end of the start bit or at the beginning of the stop bit. $55 is "more worst case". A string of *these* looks like a square wave on the oscilloscope. > Re correct value but right value of capacitor. A non polarised capacitor > is liable to be superior to a polarised one but a polarised one is liable > to be entirely OK here except at the extreme limits of operation. For such a lengthy post, you sure are being vague. What limits do you have in mind? > usually the manufacturer will (probably) spec the caps so that something > else will fail first. Why should the caps fail at all? The only other thing in the circuit is the chip itself. > Any modern non polarised cap is liable to be OK as a reservoir. The pump > caps are somewhat more sensitive to characteristics than the reservoirs > as they take switch the entire pump energy rather than just smoothing the > variations. Sensitive to which characteristics? Also, "entire pump energy" is a nebulous concept. Except for the startup transient, all currents throughout the circuit (both DC and ripple) are proportional to load current. > Again, use of a 'scope will tell you whether things areas they ought to > be. What are we looking for? Generally, you just want to follow the chip manufacturer's recommendations (usually 0.1 to 1.0 uF for both pump and reservoir capacitors). If you find that you're overloading the outputs, you aren't using the right technology for the job. RS-232 isn't really meant to go much over 10 mA, so a source impedance of 100 ohms (e.g., 0.1 uF pump caps switching at 100 kHz) is plenty low, giving you a 1 volt droop at full output current. Excessive droop simply tells us that the output impedance of the charge pump is too high for the load. Since the switching frequency is usually fixed, the only thing you can do to lower the impedance is to increase the size of the pump caps. But if you make the caps too large, the resistance and/or saturation current of the pump switches begins to dominate, increasing their power dissipation. -- Dave Tweed -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu