PIC18F to PIC24F Migration: An Overview - Microchip

TWO-SPEED START-UPTwo-Speed Start-up is implemented identically inPIC18F and PIC24F devices. In both cases, the featureis controlled by the IESO Configuration bit.FAIL-SAFE CLOCK MONITORThe Fail-Safe Clock Monitor feature is also available forthe PIC24F. It is controlled, along with run-time clockswitching, by the Configuration Word bits,FCKSM1:FCKSM0 (CW2). FSCM in PIC24Fdevices is similar to the PIC18F implementation byautomatically switching to the FRC when the primaryoscillator stops.The single significant difference is the secondaryeffects of an FSCM event. For PIC18F devices, aprimary oscillator failure sets the OSCFIF interrupt flagbit, which can optionally generate a device interrupt.For PIC24F devices, an FSCM event sets both the CFflag (OSCCON) and OSCFAIL (INTCON1) bits,and generates an unmaskable hardware trap whichthen must be cleared.MIGRATING A TYPICAL APPLICATION• Most applications built on a PIC18F device withnanoWatt Technology will be able to use the sameoscillator type and clock frequency when aPIC24F device is substituted. This is particularlytrue when the oscillator uses a crystal (HS modeor XT mode, between 3.5 and 10 MHz), anexternal clock generator or the internal RCoscillator block.• PIC18F applications that use a 32 kHz crystal forthe primary oscillator will need to use thesecondary oscillator for their PIC24F equivalentversions. The crystal circuit will need to be movedto the T1OSCI/T1OSCO pins, and the SecondaryOscillator mode (SOSC) will need to be selectedas the start-up clock source.• PIC18F applications using an External RCPrimary Oscillator mode, or an XT Oscillatormode running below 3.5 MHz, must now use theFRC oscillator as the default clock source.Configuration will need to be changed to selectFRC as the default start-up clock source.• If clock switching is used, the clock switchsequences will need to be added. These aredescribed in the appropriate device data sheet.MIGRATION CONSIDERATIONSWhen migrating to a PIC24F microcontroller (or anymicrocontroller, for that matter), any application that isbased on a crystal clock source should be re-evaluatedfor oscillator operation and stability. It is important toverify that the crystal performance is reliable across thevoltage, temperature and process variationsanticipated for the application.For more information, refer to the application noteslisted in the “References” section on page 45.DS39764A-page 22© 2006 Microchip Technology Inc.