COM20020D datasheet(13/72 Pages) SMSC

13 / 72 page

SYSTEM DESCRIPTION

MICROCONTROLLER INTERFACE

The top halves of Figures 2 and 3 illustrate typical

COM20020D interfaces to the microcontrollers.

The interfaces consist of a 8-bit data bus, an

address bus and a control bus. In order to support

a wide range of microcontrollers without requiring

glue logic and without increasing the number of

pins, the COM20020D automatically detects and

adapts to the type of microcontroller being used.

Upon hardware reset, the COM20020D first

determines whether the read and write control

signals are separate READ and WRITE signals

(like the 80XX) or DIRECTION and DATA

STROBE (like the 68XX). To determine the type of

control signals, the device requires the software to

execute at least one write access to external

memory

before

attempting

access

the

COM20020D.

The device defaults to 80XX-like

signals.

Once the type of control signals are

determined, the COM20020D remains in this

interface mode until the next hardware reset

occurs.

The

second

determination

the

COM20020D makes is whether the bus is

multiplexed or non-multiplexed. To determine the

type of bus, the device requires the software to

write to an odd memory location followed by a

read from an odd location before attempting to

access the COM20020D. The signal on the A0 pin

during

the

odd

location

access

tells

the

COM20020D the type of bus. Since multiplexed

operation requires A0 to be active low, activity on

the A0 line tells the COM20020D that the bus is

non-multiplexed.

The

device

defaults

multiplexed operation. Both determinations may

be made simultaneously by performing a WRITE

followed by a READ operation to an odd location

within the COM20020D Address space 20020D

registers. Once the type of bus is determined, the

COM20020D remains in this interface mode until

hardware reset occurs.

Whenever nCS and nRD are activated, the preset

determinations are assumed as final and will not

be changed until hardware reset. Refer to

Description of Pin Functions section for details on

the related signals. All accesses to the internal

RAM and the internal registers are controlled by

the COM20020D. The internal RAM is accessed

via

pointer-based

scheme

(refer

the

Sequential Access Memory section), and the

internal

registers

are

accessed

via

direct

addressing. Many peripherals are not fast enough

to take advantage of high-speed microcontrollers.

Since microcontrollers do not typically have

READY

inputs,

standard

peripherals

cannot

extend cycles to extend the access time.

The

access time of the COM20020D, on the other

hand, is so fast that it does not need to limit the

speed of the microcontroller. The COM20020D is

designed to be flexible so that it is independent of

the microcontroller speed.

The COM20020D provides for no wait state

arbitration via direct addressing to its internal

registers and a pointer based addressing scheme

to access its internal RAM. The pointer may be

used

auto-increment

mode

for

typical

sequential buffer emptying or loading, or it can be

taken out of auto-increment mode to perform

random accesses to the RAM. The data within

the RAM is accessed through the data register.

Data being read is prefetched from memory and

placed into the data register for the microcontroller

to read.

It is important to notice that only by

writing a new address pointer (writing to an

address pointer low), one obtains the contents of

COM20020D internal RAM. Performing only read

from the Data Register does not load new data

from the internal RAM. During a write operation,

the data is stored in the data register and then

written into memory. Whenever the pointer is

loaded for reads with a new value, data is

immediately prefetched to prepare for the first

read operation.

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