COM20020ILJP datasheet(49/72 Pages) MICROCHIP

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5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM

Datasheet

SMSC COM20020I Rev D

Page 49

Revision 12-05-06

DATASHEET

RCVACT=0, TOKEN=0, TXEN=1: No receive activity is seen and the basic transmit function is enabled.

The transmitter and/or receiver are not functioning properly.

RCVACT=0, TOKEN=0, TXEN=0: No receive activity and basic transmit function disabled. This node is

not connected to the network.

The Excessive NAK (EXCNAK) bit is used to replace a timeout function traditionally implemented in

software. This function is necessary to limit the number of times a sender issues a FBE to a node with no

available buffer. When the destination node replies to 128 FBEs with 128 NAKs or 4 FBEs with 4 NAKs,

the EXCNAK bit of the sender is set, generating an interrupt. At this point the software may abandon the

transmission via the "Disable Transmitter" command. This sets the TA bit to logic "1" when the node next

receives the token, to allow a different transmission to occur. The timeout value for the EXNACK bit (128

or 4) is determined by the FOUR-NAKS bit on the Setup1 Register.

The user may choose to wait for more NAK's before disabling the transmitter by taking advantage of the

wraparound counter of the EXCNAK bit. When the EXCNAK bit goes high, indicating 128 or 4 NAKs, the

"POR Clear Flags" command maybe issued to reset the bit so that it will go high again after another count

of 128 or 4. The software may count the number of times the EXCNAK bit goes high, and once the final

count is reached, the "Disable Transmitter" command may be issued.

The New Next ID bit permits the software to detect the withdrawal or addition of nodes to the network.

The Tentative ID bit allows the user to build a network map of those nodes existing on the network. This

feature is useful because it minimizes the need for human intervention. When a value placed in the

Tentative ID Register matches the Node ID of another node on the network, the TENTID bit is set, telling

the software that this NODE ID already exists on the network. The software should periodically place

values in the Tentative ID Register and monitor the New Next ID bit to maintain an updated network map.

6.9

Oscillator

The COM20020ID contains circuitry which, in conjunction with an external parallel resonant crystal or TTL

clock, forms an oscillator.

If an external crystal is used, two capacitors are needed (one from each leg of the crystal to ground). No

external resistor is required, since the COM20020ID contains an internal resistor. The crystal must have

an accuracy of 0.020% or better. The oscillation frequency range is from 10 MHz to 20 MHz.

The crystal must have an accuracy of 0.010% or better when the internal clock multiplier is turned on. The

oscillation frequency must be 20MHz when the internal clock multiplier is turned on.

The XTAL2 side of the crystal may be loaded with a single 74HC-type buffer in order to generate a clock

for other devices.

The user may attach an external TTL clock, rather than a crystal, to the XTAL1 signal. In this case, a

390

Ω pull-up resistor is required on XTAL1, while XTAL2 should be left unconnected.

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