COM20020_00 Datasheet, PDF (31/82 Pages) SMSC Corporation – Universal Local Area Network Controller with 2K x 8 On-Board RAM

Next ID Register
The Next ID Register is an 8-bit, read-only
register, accessed when the sub-address bits are
set up accordingly (please refer to the
Configuration Register and SUB ADR Register).
The Next ID Register holds the value of the Node
ID to which the COM20020 will pass the token.
When used in conjunction with the Tentative ID
Register, the Next ID Register can provide a
complete network map. The Next ID Register is
updated each time a node enters/leaves the
network or when a network reconfiguration occurs.
Each time the microsequencer updates the Next
ID Register, a New Next ID interrupt is generated.
This bit is cleared by reading the Next ID Register.
Default value is 0000 0000 upon hardware or
software reset.
Status Register
The COM20020 Status Register is an 8-bit read-
only register. All of the bits, except for bits 5 and
6, are software compatible with previous SMSC
ARCNET devices. In previous SMSC ARCNET
devices the Extended Timeout status was
provided in bits 5 and 6 of the Status Register. In
the COM20020, the COM20020, the COM90C66,
and the COM90C165, COM20020-5, COM20051
and COM20051+ these bits exist in and are
controlled by the Configuration Register. The
Status Register contents are defined as in Table
4, but are defined differently during the Command
Chaining operation. Please refer to the Command
Chaining section for the definition of the Status
Register during Command Chaining operation.
The Status Register defaults to the value 1XX1
0001 upon either hardware or software reset.
Register represent different situations. All of
these bits, except the Excessive NAcK bit and the
New Next ID bit, are reset to logic "0" upon
reading the Diagnostic Status Register or upon
software or hardware reset. The EXCNAK bit is
reset by the "POR Clear Flags" command or upon
software or hardware reset. The Diagnostic
Status Register defaults to the value 0000 000X
upon either hardware or software reset.
Command Register
Execution of commands are initiated by
performing microcontroller writes to this register.
Any combinations of written data other than
those listed in Table 5 are not permitted and may
result in incorrect chip and/or network operation.
Address Pointer Registers
These read/write registers are each 8-bits wide
and are used for addressing the internal RAM.
New pointer addresses should be written by first
writing to the High Register and then writing to the
Low Register because writing to the Low Register
loads the address. The contents of the Address
Pointer High and Low Registers are undefined
upon hardware reset. Writing to Address Pointer
low loads the address.
Configuration Register
The Configuration Register is a read/write register
which is used to configure the different modes of
the COM20020. The Configuration Register
defaults to the value 0001 1000 upon hardware
reset only. SUBAD0 and SUBAD1 point to the
selection in Register 7.
Diagnostic Status Register
The Diagnostic Status Register contains seven
read-only bits which help the user troubleshoot the
network or node operation. Various combinations
of these bits and the TXEN bit of the Configuration
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