Intel mcs-48 manual Oscillator and Clock, Timer/Event Counter

SINGLE COMPONENT SYSTEM

2.7 Oscillator and Clock

The 8021 contains its own onboard oscilla- tor and clock circuit, requiring only an external timing control element. This con- trol element can be a crystal, inductor, or clock in. The capacitor normally required in inductor timing control operation is inte- grated onto the 8021. All internal time slots are derived from the external element, and all outputs are a function of the oscillator frequency. Pins XTAL1 and XTAL2 are used to input the particular control element. An instruction cycle consists of 10 states, and each state is a time slot of 3 oscillator periods. (See Figure 2) Therefore, to obtain a 10 Msec instruction cycle, a 3M Hz crystal should be used. An oscillator frequency of approximately 3MHz may also be obtained by connecting a 470 MH inductor between XTAL1 and XTAL2. Note that the required inductance may vary and should be adjust- ed as necessary.

The 8021 utilizes dynamic RAM and certain other dynamic logic. Due to the clocking required with dynamic circuits, the oscil- lator frequency must be equal to or greater than 600K Hz, or improper operation may occur.

2.8 Timer/Event Counter

The 8021 has internal timer/event counter circuits that can monitor elapsed time or count external events that occur during program execution. The circuit has an 8-bit binary up-counter that is presettable and readable with two MOV instructions. These instructions transfer the contents of the accumulator to the counter and vice-versa. The counter content is not affected by Reset, and is initialized solely by the MOV r,A instruction. The counter is stopped by a RESET or STOP TCNT instruction and remains stopped until started as a timer by a STRT T instruction or as an event counter by a STRT CNT instruction. Once started,

CYCLE	r------C1 •	'I'	C2	1
TIMES	I 1 I 2 I 3 I 4 I 5 I 6 I 7 I 8	I 9 I 10 I 1	5 I 6 I 7 I 8 I 9 I 10 I
ALE	n		n
	------~ ~------------~ ~------
		IN	>C
		DATA

PO,P1,P2_______-- JX'- DA_T_AO_U_T______________

All PORTS FOR IN AND OUTl INSTRUCTIONS

FIGURE 2. 8021 TIMING DIAGRAM

2·21