COM20020IP3V datasheet(17/82 Pages) SMSC

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High Speed CPU Bus Timing Support

High speed CPU bus support was added to the

COM20020.

The reasoning behind this is as

follows:

With

the

Host

interface

Non-

multiplexed Bus mode, I/O address and Chip

Select signals must be stable before the read

signal is active and remain after the read signal

is inactive. But the High Speed CPU bus timing

doesn't adhere to these timings. For example, a

RISC type single chip microcontroller (like the

HITACHI SuperH series) changes I/O address at

the same time as the read signal. Therefore,

several external logic ICs would be required to

connect to this microcontroller.

In addition, the Diagnostic Status (DIAG) register

is cleared automatically by reading itself. The

internal DIAG register read signal is generated

by decoding the Address (A2-A0), Chip Select

(nCS) and Read (nRD) signals. The decoder will

generate a noise spike at the above tight timing.

The DIAG register is cleared by the spike signal

without

reading

itself.

This

unexpected

operation. Reading the internal RAM and Next

Id Register have the same mechanism as

reading the DIAG register.

Therefore,

the

address

decode

and

host

interface mode blocks were modified to fit the

above CPU interface to support high speed CPU

bus timing.

In Intel CPU mode (nRD, nWR

mode), 3 bit I/O address (A2-A0) and Chip Select

(nCS) are sampled internally by Flip-Flops on the

falling edge of the internal delayed nRD signal.

The internal real read signal is the more delayed

nRD signal. But the rising edge of nRD doesn't

delay. By this modification, the internal real

address and Chip Select are stable while the

internal real read signal is active. Refer to figure

4 below.

FIGURE 4 - HIGH SPEED CPU BUS TIMING - INTEL CPU MODE

The I/O address and Chip Select signals, which

are supplied to the data output logic, are not

sampled. Also, the nRD signal is not delayed,

because the above sampling and delaying paths

decrease the data access time of the read cycle.

A2-A0, nCS

nRD

Delayed nRD

(nRD1)

Sampled A2-A0, nCS

More delayed nRD

(nRD2)

VALID

COM20020IP3V Datasheet(PDF) 17 Page - SMSC Corporation

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