COM20020D Datasheet, PDF (34/72 Pages) SMSC Corporation – ULANC Revision D Universal Local Area Network Controller with 2K x 8 On-Board RAM

Table 11 - Setup 1 Register
BIT
BIT NAME
SYMBOL
DESCRIPTION
7 Pulse1 Mode
P1MODE
This bit determines the type of PULSE1 output driver used
in Backplane Mode. When high, a push/pull output is used.
When low, an open drain output is used. The default is
open drain.
6 Four NACKS
FOUR
NACKS
This bit, when set, will cause the EXNACK bit in the
Diagnostic Status Register to set after four NACKs to Free
Buffer Enquiry are detected by the COM20020D. This bit,
when reset, will set the EXNACK bit after 128 NACKs to
Free Buffer Enquiry. The default is 128.
5 Reserved
Do not set.
4 Receive All
RCVALL
3,2,1 Clock Prescaler Bits CKP3,2,1
3,2,1
This bit, when set, allows the COM20020D to receive all
valid data packets on the network, regardless of their
destination ID. This mode can be used to implement a
network monitor with the transmitter on- or off-line. Note
that ACKs are only sent for packets received with a
destination ID equal to the COM20020D's programmed
node ID. This feature can be used to put the COM20020D
in a 'listen-only' mode, where the transmitter is disabled and
the COM20020D is not passing tokens. Defaults low.
These bits are used to determine the data rate of the
COM20020D. The following table is for a 20 MHz crystal:
(Clock Multiplier is bypassed)
CKP3 CKP2 CKP1 DIVISOR
SPEED
0
0
0
8
2.5Mbs
0
0
1
16
1.25Mbs
0
1
0
32
625Kbs
0
1
1
64
312.5Kbs
1
0
0
128
156.25Kbs
NOTE: The lowest data rate achievable by the
COM20020D is 156.25Kbs. Defaults to 000 or 2.5Mbs. For
Clock Multiplier output clock speed greater than 20 MHz,
CKP3, CKP2 and CKP1 must all be zero.
0 Slow Arbitration
Select
SLOWARB
This bit, when set, will divide the arbitration clock by 2.
Memory cycle times will increase when slow arbitration is
selected.
NOTE: For clock multiplier output clock speeds greater
than 40 MHz, SLOWARB must be set. Defaults to low.
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