Intel mcs-48 manual 8212

8212

III.BI-Dlrectlonal BUB Driver

Apair of 8212'swired (back-to-back) can be used as a symmetrical drive, bi-directional bus driver. The devices are controlled by the data bus input control which is connected to DS1 on the first 8212 and to DS2 on the second. One device is active, and acting as a straight through buffer the other is in 3-state mode. This is a very useful circuit in small system design.

IV. Interrupting Input Port

This use of an 8212 is that of a system input port that accepts a strobe from the system input source, which in turn clears the service request flip-flop and interrupts the processor. The processor then goes through a service routine, identifies the port, and causes the device selection logic to go true -enabling the system input data onto the data bus.

V.Interrupt Instruction Port

The 8212 can be used to gate the interrupt instruction, normally RESTART instructions, onto the data bus. The device is enabled from the interrupt acknowledge signal from the microprocessor and from a port selection signal. This signal is normally tied to ground. (DS1 could be used to multiplex a variety of interrupt instruction ports onto a common bus).

BI-DIRECTIONAL BUS DRIVER

J.

STB

DATA		I		"-	,.l	DATA
				v
BUS						BUS
				8212
		~	:.
				--c CLR
DATA BUS		-		Y G~D	L--
CONTROL
(0= L -	RJ
(I = R -	LJ

STB

8212 ~

LCLR "

----..J f---Y

	GND
INTERRUPTING INPUT PORT
		DATA
	INPUT	BUS
	STROBE
	STB
	SYSTEM
	INPUT
	SYSTEM
	RESET
PORT	{

SELECTION

(DS1·DS2J ------- ' t -----T~c~~0~R~6~JCKT

OR

TO CPU INTERRUPT INPUT

INTERRUPT INSTRUCTION PORT

DATA

BUS

STB

RESTART INSTRUCTION (RST 0 - RST 7J

(DSIJ PORT SELECTION

INTERRUPT ACKNOWLEDGE _ ........___--l

8-27 AFN·OO731A·04