5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM
Table 11 - Setup 1 Register
SYMBOL DESCRIPTION
BIT
BIT NAME
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 COM20020I. This bit,
when reset, will set the EXNACK bit after 128 NACKs to
Free Buffer Enquiry. The default is 128.
5
4
Reserved
Do not set.
Receive All
RCVALL
This bit, when set, allows the COM20020I 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 COM20020I's programmed node
ID. This feature can be used to put the COM20020I in a
'listen-only' mode, where the transmitter is disabled and the
COM20020I is not passing tokens. Defaults low.
3,2,1 Clock Prescaler Bits CKP3,2,1
These bits are used to determine the data rate of the
COM20020I. The following table is for a 20 MHz crystal:
(Clock Multiplier is bypassed)
3,2,1
CKP3 CKP2 CKP1 DIVISOR
SPEED
2.5Mbs
0
0
0
0
1
0
0
1
1
0
0
1
0
1
0
8
16
32
64
128
1.25Mbs
625Kbs
312.5Kbs
156.25Kbs
NOTE: The lowest data rate achievable by the COM20020I
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
Select
Arbitration 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.
SMSC COM20020I 3.3V
Page 33
Revision 12-06-06
DATASHEET
SMSC [ SMSC CORPORATION ]
