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COM2002IP Datasheet(PDF) 9 Page - SMSC Corporation |
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COM2002IP Datasheet(HTML) 9 Page - SMSC Corporation |
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9 / 72 page  9 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) 0 0 20 MHz (Up to 2.5Mbps) Default (Bypass) 0 1 40 MHz (Up to 5Mbps) 1 0 Reserved 1 1 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 "sec (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. The EF bit in the SETUP2 register must be set when the data rate is over 5 Mbps. NETWORK RECONFIGURATION A significant advantage of the COM20020 is its ability to adapt to changes on the network. Whenever a new node is activated or deactivated, a NETWORK RECONFIGURATION is performed. When a new COM20020 is turned on (creating a new active node on the network), or if the COM20020 has not received an INVITATION TO TRANSMIT for 420mS, or if a software reset occurs, the COM20020 causes a NETWORK RECONFIGURATION by sending a RECONFIGURE BURST consisting of eight marks and one space repeated 765 times. The purpose of this burst is to terminate all activity on the network. Since this burst is longer than any other type of transmission, the burst will interfere with the next INVITATION TO TRANSMIT, destroy the token and keep any other node from assuming control of the line. When any COM20020 senses an idle line for greater than 41 "S, which occurs only when the token Is lost, each COM20020 starts an internal timeout equal to 73 "s times the quantity 255 minus its own ID. The COM20020 starts network reconfiguration by sending an invitation to transmit first to itself and then to all other nodes by decrementing the destination Node ID. If the timeout expires with no line activity, the COM20020 starts sending INVITATION TO TRANSMIT with the Destination ID (DID) equal to the currently stored NID. Within a given network, only one COM20020 will timeout (the one with the highest ID number). After sending the INVITATION TO TRANSMIT, the COM20020 waits for activity on the line. If there is no activity for 37.4 "S, the COM20020 increments the NID value and transmits another INVITATION TO TRANSMIT using the NID equal to the DID. If activity appears before the 37.4 "S timeout expires, the COM20020 releases control of the line. During NETWORK RECONFIGURATION, INVITATIONS TO TRANSMIT are sent to all NIDs (1-255). Each COM20020 on the network will finally have saved a NID value equal to the ID of the COM20020 that it released control to. At this point, control is passed directly from one node to the next with no wasted INVITATIONS TO TRANSMIT being sent to ID's not on the network, until the next NETWORK RECONFIGURATION occurs. When a node is powered off, the previous node attempts to pass the token to it by issuing an |
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