COM20019 Datasheet, PDF (35/81 Pages) SMSC Corporation – Low Cost ARCNET (ANSI 878.1) 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 COM20019. 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
This bit, when set, allows the COM20019 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
COM20019's programmed node ID. This feature can be
used to put the COM20019 in a 'listen-only' mode, where
the transmitter is disabled and the COM20019 is not
passing tokens. Defaults low.
3,2,1 Clock Prescaler Bits CKP3,2,1
3,2,1
These bits are used to determine the data rate of the
COM20019. The following table is for a 20 MHz crystal:
CKP3 CKP2 CKP1 DIVISOR
0
1
1
64
SPEED
312.5 Kbs
1
0
0
128
156.25 Kbs
NOTE: The lowest data rate achievable by the COM20019
is 156.25 Kbs. Defaults to 011 or 312.5 Kbps.
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.
Defaults to low.
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