Intel 80C196NU, Microcontroller manual Internal Timing, Clock Circuitry 8XC196NP

ARCHITECTURAL OVERVIEW

2.4INTERNAL TIMING

The clock circuitry of the 8XC196NP (Figure 2-3) is identical to that of earlier MCS 96 micro- controllers. It receives an input clock signal on XTAL1 provided by an external crystal or clock and divides the frequency by two. The clock generators accept the divided input frequency from the divide-by-two circuit and produce two nonoverlapping internal timing signals, PH1 and PH2. These signals are active when high.

XTAL1

XTAL2

Disable Clock Input

(Powerdown)

FXTAL1
			Divide-by-two
			Circuit		Disable Clocks
					(Powerdown)
						Peripheral Clocks (PH1, PH2)
			Clock
						CLKOUT
		Disable	Generators
		Oscillator				CPU Clocks (PH1, PH2)
	(Powerdown)
					Disable Clocks
				(Idle, Powerdown)
						A3161-01

Figure 2-3. Clock Circuitry (8XC196NP)

The 80C196NU’s clock ci rcuitry (Figure 2-4) implements phase-locked loop and clock multiplier circuitry, which can substantially increase the CPU clock rate while using a lower-frequency in- put clock. The clock circuitry accepts an input clock signal on XTAL1 provided by an external crystal or oscillator. Depending on the values of the PLLEN1 and PLLEN2 pins, this frequency is routed either through the phase-locked loop and multiplier or directly to the divide-by-two cir- cuit. The multiplier circuitry can double or quadruple the input frequency (FXTAL1) before the fre- quency (f) reaches the divide-by-two circuitry. The clock generators accept the divided input frequency (f/2) from the divide-by-two circuit and produce two nonoverlapping internal timing signals, PH1 and PH2. These signals are active when high.

NOTE

For brevity, this manual uses lowercase “f” to represent the internal clock frequency of both the 8XC196NP and the 80C196NU. For the 8XC196NP, f is

equal to FXTAL1. For the 80C196NU, f is equal to either FXTAL1, 2FXTAL1, or

4FXTAL1, depending on the clock multiplier mode, which is controlled by the PLLEN1 and PLLEN2 input pins.

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