Post by thread:External Clocking of 16F84

Hi all, I need to externally clock a 'F84-04 with a 4 MHz clock. Does the duty cycle of this clock need to be exactly 50%. I cannot find any reference to Duty Cycle requirements in the data sheet, and my feeling is that it is not critical???? Also when programming the PIC I assume I still select "XT" for the crystal mode when supplying an external clock into the CLKIN pin?? thanks for your help Rob Perth Western Australia

At 11:23 04/14/99 +0800, Rob wrote: >I need to externally clock a 'F84-04 with a 4 MHz clock. Does the duty >cycle of this clock need to be exactly 50%. I cannot find any reference >to Duty Cycle requirements in the data sheet, and my feeling is that it >is not critical???? they specify a minimum high and low time. as the minimum frequency can be 0 (dc), ie. the maximum high or low time approach infinity, i guess there can't be a duty cycle req. beyond the minimum hi and lo times. ge

Rob wrote: > > I need to externally clock a 'F84-04 with a 4 MHz clock. Does the duty > cycle of this clock need to be exactly 50%. I cannot find any reference > to Duty Cycle requirements in the data sheet, and my feeling is that it > is not critical???? Look for the spec in a table labelled "External Clock Timing Requirements" or something similar. There is no "duty cycle" spec, but one for a minimum "external clock in high or low time". > > Also when programming the PIC I assume I still select "XT" for the > crystal mode when supplying an external clock into the CLKIN pin?? > You can set it to either XT, LP, or HS. All this fuse does is set the value of the internal feedback resistor between OSC1 and OSC2. Since an external clock is only fed into OSC2, the feedback resistor is not used. However, you'll get lower power operation with the fuse set to LP (higher feedback resistor value). The fuse setting *will* affect the minimum external clock input high/low time spec, though. --Matt