Intel 8XC251SB, 8XC251SA, 8XC251SP, 8XC251SQ, Embedded Microcontroller Clock and Reset Unit, XTAL1

8XC251SA, SB, SP, SQ USER’S MANUAL

2.2.2Clock and Reset Unit

The timing source for the 8XC251Sx can be an external oscillator or an internal oscillator with an external crystal/resonator (see Chapter 11, “Minimum Hardware Setup”). The basic unit of time in MCS 251 microcontrollers is the state time (or state), which is two oscillator periods (see Figure 2-3). The state time is divided into phase 1 and phase 2.

The 8XC251Sx peripherals operate on a peripheral cycle, which is six state times. (This periph- eral cycle is particular to the 8XC251Sx and not a characteristic of the MCS 251 architecture.) A one-clock interval in a peripheral cycle is denoted by its state and phase. For example, the PCA timer is incremented once each peripheral cycle in phase 2 of state 5 (denoted as S5P2).

The reset unit places the 8XC251Sx into a known state. A chip reset is initiated by asserting the RST pin or allowing the watchdog timer to time out (see Chapter 11, “Minimum Hardware Set- up”).

				Phase 1		Phase 2
					P1		P2
	XTAL1
				TOSC
				2 TOSC = State Time
State 1		State 2		State 3		State 4		State 5		State 6
P1	P2	P1	P2	P1	P2	P1	P2	P1	P2	P1	P2
XTAL1
				Peripheral Cycle
											A2604-02

Figure 2-3. Clocking Definitions

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