COM20020I_0609 Datasheet, PDF(27/69 Page) SMSC Corporation – 5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM

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COM20020I_0609 Datasheet, PDF (27/69 Pages) SMSC Corporation – 5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM

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5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM

The sub-address register is new to the COM20020I 3V, 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

Setup 2 Register

The Setup 2 Register is new to the COM20020I 3V. 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 CKUP bit 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 COM20020I 3V (if EF = 0, the timing is the same as in the COM20020I 3V Rev. B). See

Appendix âAâ. 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 âAâ.

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.

Note*:

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*

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

SMSC COM20020I 3.3V Rev.E

Page 27

DATASHEET

Revision 09-11-06

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