COM20020IP3V datasheet(32/82 Pages) SMSC

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Sub-Address Register

The

sub-address

new

the

COM20020, previously a reserved register. Bits 2,

1 and 0 are used to select one of the registers

assigned to address 7h. SUBAD1 and SUBAD0

already exist in the Configuration register on the

COM20020B. They are exactly same as those in

the Sub-Address register. If the SUBAD1 and

SUBAD0 bits in the Configuration register are

changed, the SUBAD1and SUBAD0 in the Sub-

Address register are also changed. SUBAD2 is a

new sub-address bit. It Is used to access the 1

new Set Up register, SETUP2. This register is

selected by setting SUBAD2=1. The SUBAD2 bit

cleared

automatically

writing

the

Configuration register.

Setup 1 Register

The Setup 1 Register is a read/write 8-bit register

accessed when the Sub Address Bits are set up

accordingly

(see the bit definitions of the

Configuration Register). The Setup 1 Register

allows the user to change the network speed (data

rate) or the arbitration speed independently,

invoke the Receive All feature and change the

nPULSE1 driver type.

The data rate may be

slowed to 156.25Kbps and/or the arbitration speed

may be slowed by a factor of two. The Setup 1

hardware reset only.

Setup 2 Register

The Setup 2 Register is new to the COM20020.

It is an 8-bit read/write register accessed when

the Sub Address Bits SUBAD[2:0] are set up

accordingly (see the bit definitions of the Sub

Address Register). This register contains bits for

various functions. The CKUP1,0 bits select the

clock to be generated from the 20 MHz crystal.

The RBUSTMG bit is used to Disable/Enable

Fast Read function for High Speed CPU bus

support. The EF bit is used to enable the new

timing for certain functions in the COM20020 (if

EF = 0, the timing is the same as in the

COM20020 Rev. B). See Appendix “A”.

The

NOSYNC bit is used to enable the NOSYNC

function during initialization. If this bit is reset,

the line has to be idle for the RAM initialization

sequence to be written. If set, the line does not

have to be idle for the initialization sequence to

be written. See Appendix “A”.

The RCNTM[1,0] bits are used to set the time-

out period of the recon timer. Programming this

timer for shorter time periods has the benefit of

shortened network reconfiguration periods. The

time periods shown in the table on the following

page are limited by a maximum number of nodes

in the network. These time-out period values are

for 5Mbps. For other data rates, scale the time-

out period time values accordingly; the maximum

node count remains the same.

RCNTM1

RCNTM0

TIME-OUT

PERIOD

MAX NODE

COUNT

420 mS

Up to 255 nodes

105 mS

Up to 64 nodes

52.5 mS

Up to 32 nodes

26.25 mS*

Up to 16 nodes*

Note*: The node ID value 255 must exist in the network for the 26.25 mS time-out to be valid.

COM20020IP3V 数据表(PDF) 32 Page - SMSC Corporation

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