User's Manual

30
Table 10 - Setup 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 COM20020. This bit, when reset, will set the
EXNACK bit after 128 NACKs to Free Buffer Enquiry. The default
is 128.
5 ET3 ET3 This bit, when set, scales down protocol timeout values of
Response Time and Idle Time but not Reconfiguration Time to
optimize network performance in short topologies. Provides a
scaling factor of ÷ 3. Defaults to a zero. Must be reset to be
ARCNET compliant.
4 Receive All RCVALL 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.
3,2,1
Clock Prescaler Bits
3,2,1
CKP3,2,1 These bits are used to determine the data rate of the COM20020.
The following table is for a 20MHz crystal:
CKP3
0
0
0
0
1
1
1
1
CKP2
0
0
1
1
0
0
1
1
CKP1
0
1
0
1
0
1
0
1
DIVISOR
8
16
32
64
128
256
SPEED
2.5Mbs
1.25Mbs
625Kbs
312.5Kbs
156.25Kbs
Reserved
Reserved
Reserved
NOTE: The lowest data rate achievable by the COM20020 is
156.25Kbs. A divide by 256 is provided for those systems that use
faster clock speeds. Defaults to 000 or 2.5Mbs.
0 Slow Arbitration Select SLW-ARB This bit, when set, will divide the arbitration clock by 2. Memory
cycle times will increase when slow arbitration is selected.
NOTE: For clock speeds greater than 20MHz, SLOWARB must be
set. Defaults to low.