MC92501 Datasheet, PDF (21/52 Pages) Motorola, Inc – ATM Cell Processor

SECTION 8. IMPROVED HOST INTERFACE
8.1. Overview
In order to improve the MC92501 interface to the
microprocessor, the following features were added:
8.1.1. An Additional MDTACK Signal
The MDTACK signals enable a glueless interface to systems
on which there are two MDTACK signals and their combination
conveys the bus width of the slave. The MDTACK1 signal is
driven only when the MDTACK0 signal is driven and when the
MDCÑMDTACK Drive Control is set.
8.1.2. Programmable MREQ Signals
The MC92500 generates three DREQ signals: EMMREQ
for external memory request, MCIREQ for cell insertion
request, and MCOREQ for cell extraction request. The
MC92501 changes the functionality (and the name) of the
three DREQ lines. Each MREQ[n] line may be connected to
each of the internal requests: external memory, cell insertion,
or cell extraction. If a system contains three DMAs, then each
DMA may be connected to a different DMA DREQ signal. In
systems with less than three DMAs, each such DMA should
switch between the MREQ lines. This switching can be done
when using the MC92500 through the use of glue logic. In
these cases, this feature enables a glueless interface. The
MREQ signals default to the MC92500 conÞguration so that
backwards compatibility is maintained.
8.1.3.
Update the Definition of MWSH and MWSL
Signals
MWSH and MWSL are used in MC92500 as word select
during external memory accesses. Some systems assert
these signals only during write cycles. The MC92501 uses
these signals only on external memory write accesses. Read
accesses are always performed regardless of these signals.
Moreover, another use of these signals is for systems to use
them for Address[1] bit and SIZE. The MC92501 supports
another deÞnition for the same pins: MWSH pin can serve as
Address[1] while MWSL can serve as SIZE. The WSSMÑ
Word Select Signals Mode bit selects which mode is
supported. Table 2 describes the pinsÕ functionality:
Table 2. Host Interface Fields
WSSMÑWord Select Signals
Mode = 0
MWSH
0
0
1
MWSL
0
1
0
WSSMÑWord Select Signals
Mode = 1
and
DOÑData Order = 0
A1
Size
x
0
0
1
1
1
WSSMÑWord Select Signals
Mode = 1
and
DOÑData Order = 1
A1
Size
x
0
1
1
0
1
Function
Write D(31:0)
Write D(31:16)
Write D(15:00)
MOTOROLA
MC92501
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