5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM
Datasheet
6.2.12 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.
6.2.13 Setup 2 Register
The Setup 2 Register is new to the COM20020ID. 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 COM20020ID (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, pg 68.
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
PERIOD
MAX NODE
COUNT
RCNTM1 RCNTM0
0
0
1
1
0
1
0
1
420 mS
Up to 255 nodes
Up to 64 nodes
Up to 32 nodes
Up to 16 nodes
Note 6.3
105 mS
52.5 mS
26.25 mS*
Note 6.3
The node ID value 255 must exist in the network for the 26.25 mS time-out to be valid.
Revision 12-05-06
Page 30
SMSC COM20020I Rev D
DATASHEET
SMSC [ SMSC CORPORATION ]
