On Tue, 19 May 1998 11:02:31 -0600 Matt Bonner writes: >Steve Baldwin wrote: >> >> Probably not, although you would really need to have the full specs >> from the resonator manufacturer to be sure. It would be nice to have >> a bit more info from Microchip on what the oscillator mode switches >> do. Do they alter just the frequency response of the amp, the gain, >> the o/p impedance or a mixture ? >> >I asked MChip directly about this last year. Apparently, changing the >oscillator mode only changes the value of the internal feedback >resistor. One of my projects used an external 2.4576MHz oscillator - >I >programmed the PIC for LP mode (highest feedback resistance) to >achieve >the lowest power consumption. The oscillator "fuses" change the transconductance (Iout/Vin) of the oscillator, probably by switching in different sizes of MOSFET's. Early PIC16C5X data had graphs showing the transconductance for different modes. Apparently we don't need to know that now, so Mchip leaves it out. With a CMOS inverter type of oscillator such as in a PIC, the feedback resistor is needed only to control the DC operating point of the oscillator. For the RF signal, it provides undesirable negative feedback, so it should be as large as possible. Power consumption is lower with the lower speed oscillator settings because the low transconductance means less conduction between Vdd and Vss when the oscillator is in the linear range. Lower transconductance also means less output current, thus lower crystal drive. Damage to crystals is very unlikely except for low-frequency ones, in which case a resistor in series with the oscillator output pin is used. You can check for crystal overdrive by increasing the supply voltage. If the frequency changes significantly, the crystal is probably being overdriven. _____________________________________________________________________ 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]