Lou Calkins

CPU cycles vs. Instruction cycles vs. BUS cycles vs. Oscillator cycles

Discussion created by Lou Calkins on Apr 18, 2007
Latest reply on Apr 18, 2007 by Lou Calkins
I am migrating from another micro to the HCS08AW32 processor and I wanted to be sure I know how the clocks work.  From what I gather in the manual, if I have an external 4 MHz resonator, and my ICGC2 register is set to divide by 1, I think I have IGCOUT of 4 MHz, right?  The block diagram shows that connecting directly to the CPU block, so can I assume that instructions are running with an instruction cycle time of 250 nS (i.e. 4 MHz)?  Or is the 4 MHz divided down such that a CPU cycle is more than one oscillator cycle?  My CodeWarrior debugger shows what I think is BUSCLK cycles which is IGCOUT/2 or 2 MHz.  But it looks like BUSCLK is used by internal peripherals like the TPM, so I am not sure what the speed of a CPU cycle is.  Can someone get me straight on this?  Thanks