COM20019IP datasheet(38/81 Pages) SMSC

38 / 81 page

DISCONTINUED DATASHEET

INTERNAL RAM

The integration of the 2K x 8 RAM in the

COM20019 represents significant real estate

savings. The most obvious benefit is the 48 pin

package in which the device is now placed (a

direct result of the integration of RAM).

addition, the PC board is now free of the

cumbersome external RAM, external latch, and

multiplexed

address/data

bus

and

control

functions which were necessary to interface to the

RAM.

The integration of RAM represents

significant cost savings because it isolates the

system designer from the changing costs of

external RAM and it minimizes reliability problems,

assembly time and costs, and layout complexity.

Sequential Access Memory

The internal RAM is accessed via a pointer-based

scheme.

Rather than interfering with system

memory, the internal RAM is indirectly accessed

through the Address High and Low Pointer

Registers. The data is channeled to and from the

microcontroller via the 8-bit data register.

For

example: a packet in the internal RAM buffer is

read

the

microcontroller

writing

the

corresponding address into the Address Pointer

High and Low Registers (offsets 02H and 03H).

Note that the High Register should be written first,

followed by the Low Register, because writing to

the Low Register loads the address. At this point

the device accesses that location and places the

corresponding data into the data register.

The

microcontroller then reads the data register (offset

04H) to obtain the data at the specified location. If

the Auto Increment bit is set to logic "1", the

device will automatically increment the address

and place the

next byte of

data into the data

This process is continued until the entire packet

is read out of RAM.

Refer to Figure 7 for an

illustration of the Sequential Access operation.

When switching between reads and writes, the

pointer must first be written with the starting

address. At least one cycle time should separate

the pointer being loaded and the first read (see

timing parameters).

Access Speed

The COM20019 is able to accommodate very fast

access cycles

to its

registers

and buffers.

Arbitration to the buffer does not slow down the

cycle because the pointer based access method

allows data to be prefetched from memory and

stored in a temporary register. Likewise, data to

be written is stored in the temporary register and

then written to memory.

For systems which do not require quick access

time, the arbitration clock may be slowed down by

setting bit 0 of the Setup1 Register equal to logic

"1". Since the Slow Arbitration feature divides the

input clock by two, the duty cycle of the input clock

may be relaxed.

SOFTWARE INTERFACE

The microcontroller interfaces to the COM20019

via software by accessing the various registers.

These actions are described in the Internal

Registers

section.

The

software

flow

for

accessing the data buffer is based on the

Sequential Access scheme. The basic sequence

is as follows:

•

Disable Interrupts

•

Write to Pointer Register High (specifying

Auto-Increment mode)

•

Write to Pointer Register Low (this loads the

address)

•

Enable Interrupts

•

Read or Write the Data Register (repeat as

many times as necessary to empty or fill the

buffer)

•

The pointer may now be read to determine

how many transfers were completed.

COM20019IP Arkusz danych(PDF) 38 Page - SMSC Corporation

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