COM20020 Datasheet, PDF (36/72 Pages) SMSC Corporation – COM20020 3.3V ULANC Universal Local Area Network Controller with 2K x 8 On-Board RAM

Table 12 - Setup 2 Register
BIT
BIT NAME
7 Read Bus Timing
Select
6 Reserved
5,4 Clock Multiplier
3 Enhanced
Functions
SYMBOL
RBUSTMG
CKUP1, 0
EF
DESCRIPTION
This bit is used to Disable/Enable the High Speed CPU
Read function for High Speed CPU bus support.
RBUSTMG=0: Disable (Default), RBUSTMG=1: Enable. It
does not influence write operation. High speed CPU Read
operation is only for non-multiplexed bus.
This bit is undefined.
Higher frequency clocks are generated from the 20 MHz
crystal through the selection of these two bits as shown.
This clock multiplier is powered-down on default. After
changing the CKUP1 and CKUP0 bits, the ARCNET core
operation is stopped and the internal PLL in the clock
multiplier is awakened and it starts to generate the 40 MHz.
The lock out time of the internal PLL is 8"Sec typically.
After 1 mS it is necessary to write command data '18H' to
command register for re-starting the ARCNET core
operation. EF bit must be ‘1’ if the data rate is over 5Mbps.
CAUTION: Changing the CKUP1 and CKUP0 bits must be
one time or less after releasing a hardware reset.
CKUP1 CKUP0 Clock Frequency (Data Rate)
0
0
20 MHz (Up to 2.5Mbps) Default
0
1
40 MHz (Up to 5Mbps)
1
0
Reserved
1
1
Reserved
Note: After changing the CKUP1 or CKUP0 bits, it is
necessary to write a command data '18H' to the command
register. Because after changing the CKUP [1, 0] bits, the
internal operation is stopped temporarily. The writing of the
command is to start the operation.
These initializing steps are shown below.
1) Hardware reset (Power ON)
2) Change CKUP[1, 0] bit
3) Wait 1mSec (wait until stable oscillation)
4) Write command '18H' (start internal operation)
5) Start initializing routine (Execute existing software)
This bit is used to enable the new enhanced functions in the
COM20020. EF = 0: Disable (Default), EF = 1: Enable. If EF
= 0, the timing and function is the same as in the
COM20020, Revision B. See appendix “A”. EF bit must be
‘1’ if the data rate is over 5Mbps.
EF bit should be ‘1’ for new design customers.
EF bit should be ‘0’ for replacement customers.
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