Intel mcs-48 manual Bidirectional Driver, Control aatlng OlEN, CS

821818226

FUNCTIONAL DESCRIPTION

Microprocessors like the 8080 are MOS devices and are generally capable of driving a single TTL load. The same is true for MOS memory devices. While this type of drive is sufficient in small systems with few componenu, quite often it is necessary to buffer the microprocessor and memories when adding components or expanding to a multi·board system.

The 8216/8226 is a four bit bi-directional bus driver specif· ically designed to buffer microcomputer system components.

Bidirectional Driver

Each buffered line of the four bit driver consists of two separate buffers that are tri-state in nature to achieve direct bus Interface and bi-directional capability. On one side of the driver the output of one buffer and the input of another are tied together (DB), this side is used to interface to the system side components such as memories, 1/0, etc., be- cause its interface is direct TTL compatible and it has high drive (50mA). On the other side of the driver the inputs and outputs are separated to provide maximum flexibility. Of course, they can be tied together so that the driver can be used to buffer a true bi-directional bus such as the 8080 Data Bus. The DO outputs on this !ide of the driver have a special high voltage output drive capability (3.65V) so that direct interface to the 8080 and 8008 CPUs is achieved with an adequate amount of noise immunity (350mV worst case).

Control aatlng OlEN, CS

The CS input is actually a device select. When it is "high" the output drivers are all forced to their high-impedance state. When it is at "zero" the device is selected (enabled) and the direction of the data flow is determined by the OlEN input.

The OlEN input controls the direction of data flow (see Figure 1) for complete truth table. This direction control is accomplished by forcing one of the pair of buffers into its high impedance state and allowing the other to transmit its data. A simple two gate circuit is used for this function.

The 8216/8226 is a device that will reduce component count in microcomputer systems and at the same time enhance noise immunity to assure reliable, high performance op- eration.

01. ()O------ p_-+-.,

08.

DO. <>---t--G--+--'

01, o---t--t>--+---,

08,

DO,o---t--G--+--'

01, o---t--t>--+---,

08,

DO,o---t--G--+--'

01, o---t---t~-+---..	,
DO, C>---t--c.I--+----	DB,
	'

' -----.....t---- o cs

OlEN

(a)8216

01.

DO.

01,

DO,

01,

DO,

01,

DO,

(b) 8226

OlEN	CS
0	0	01 . DB
,	0	DB· 00

t-,+t+,} HIGH IMPEDANCE

Figure 1. 821818228 Logic Diagrams

8-39 AFN-00733A-Q2