5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM
5.0 PROTOCOL DESCRIPTION
5.1 Network Protocol
Communication on the network is based on a token passing protocol. Establishment of the network configuration and
management of the network protocol are handled entirely by the COM20020I's internal microcoded sequencer. A
processor or intelligent peripheral transmits data by simply loading a data packet and its destination ID into the
COM20020I's internal RAM buffer, and issuing a command to enable the transmitter. When the COM20020I next
receives the token, it verifies that the receiving node is ready by first transmitting a FREE BUFFER ENQUIRY message.
If the receiving node transmits an ACKnowledge message, the data packet is transmitted followed by a 16-bit CRC. If
the receiving node cannot accept the packet (typically its receiver is inhibited), it transmits a Negative AcKnowledge
message and the transmitter passes the token. Once it has been established that the receiving node can accept the
packet and transmission is complete, the receiving node verifies the packet. If the packet is received successfully, the
receiving node transmits an ACKnowledge message (or nothing if it is not received successfully) allowing the transmitter
to set the appropriate status bits to indicate successful or unsuccessful delivery of the packet. An interrupt mask permits
the COM20020I to generate an interrupt to the processor when selected status bits become true. Figure 1 is a flow
chart illustrating the internal operation of the COM20020I connected to a 20 MHz crystal oscillator.
5.2 Data Rates
The COM20020I is capable of supporting data rates from 156.25 Kbps to 5 Mbps. The following protocol description
assumes a 5 Mbps data rate. To attain the faster data rates, the clock frequency may be doubled by the internal clock
multiplier (see next section). For slower data rates, an internal clock divider scales down the clock frequency. Thus all
timeout values are scaled as shown in the following table:
Example: IDLE LINE Timeout @ 5 Mbps = 41 μs. IDLE LINE Timeout for 156.2 Kbps is 41 μs * 32 = 1.3 ms
INTERNAL
CLOCK
FREQUENCY
CLOCK
PRESCALER
TIMEOUT SCALING
FACTOR (MULTIPLY BY)
DATA RATE
5 Mbps
40 MHz
20 MHz
Div. by 8
Div. by 8
1
2
2.5 Mbps
Div. by 16
Div. by 32
Div. by 64
Div. by 128
1.25 Mbps
625 Kbps
4
8
312.5 Kbps
156.25 Kbps
16
32
Selecting Clock Frequencies Above 2.5 Mbps
To realize a 5 Mbps network, an external 40 MHz clock must be input. However, since 40 MHz is near the frequency
of FM radio band, it is not practical for use for noise emission reasons. Therefore, higher frequency clocks are
generated from the 20 MHz crystal as selected through two bits in the Setup2 register, CKUP[1,0] as shown below.
The selected clock is supplied to the ARCNET controller.
CKUP1
CKUP0
CLOCK FREQUENCY (DATA RATE)
20 MHz (Up to 2.5Mbps) Default (Bypass)
40 MHz (Up to 5Mbps)
0
0
1
1
0
1
0
1
Reserved
Reserved
This clock multiplier is powered-down (bypassed) on default. After changing the CKUP1 and CKUP0 bits, the
ARCNET core operation is stopped and the internal PLL in the clock generator is awakened and it starts to generate
the 40 MHz. The lock out time of the internal PLL is 8uSec typically. After more than 8 μsec (this wait time is defined
as 1 msec in this data sheet), it is necessary to write command data '18H' to the command register to re-start the
ARCNET core operation. This clock generator is called “clock multiplier”.
Changing the CKUP1 and CKUP0 bits must be one time or less after releasing hardware reset.
SMSC COM20020I 3.3V
Page 11
Revision 12-06-06
DATASHEET
SMSC [ SMSC CORPORATION ]
