COM20020_00 Datasheet, PDF (32/82 Pages) SMSC Corporation – Universal Local Area Network Controller with 2K x 8 On-Board RAM

Sub-Address Register
Setup 2 Register
The sub-address register is new to 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
is cleared automatically by 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
Register defaults to the value 0000 0000 upon
hardware reset only.
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.
TIME-OUT
MAX NODE
RCNTM1 RCNTM0
PERIOD
COUNT
0
0
420 mS
Up to 255 nodes
0
1
105 mS
Up to 64 nodes
1
0
52.5 mS
Up to 32 nodes
1
1
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.
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