COM20020_00 Datasheet, PDF (42/82 Pages) SMSC Corporation – Universal Local Area Network Controller with 2K x 8 On-Board RAM

Table 11 - Setup 1 Register
BIT
BIT NAME
7 Pulse1 Mode
SYMBOL
P1MODE
6 Four NACKS
FOUR
NACKS
5 Reserved
4 Receive All
RCVALL
3,2,1 Clock Prescaler Bits CKP3,2,1
3,2,1
0 Slow Arbitration
Select
SLOWARB
DESCRIPTION
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.
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 COM20020. This bit,
when reset, will set the EXNACK bit after 128 NACKs to
Free Buffer Enquiry. The default is 128.
Do not set.
This bit, when set, allows the COM20020 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
COM20020's programmed node ID. This feature can be
used to put the COM20020 in a 'listen-only' mode, where
the transmitter is disabled and the COM20020 is not
passing tokens. Defaults low.
These bits are used to determine the data rate of the
COM20020. The following table is for a 20 MHz crystal:
(Clock Multiplier is bypassed)
CKP3
0
0
0
0
1
CKP2
0
0
1
1
0
CKP1
0
1
0
1
0
DIVISOR
8
16
32
64
128
SPEED
2.5Mbs
1.25Mbs
625Kbs
312.5Kbs
156.25Kbs
NOTE: The lowest data rate achievable by the COM20020
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.
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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