COM20022V-HT 查看數據表（PDF） - SMSC -> Microchip

零件编号

产品描述 (功能)

生产厂家

COM20022V-HT

10Mbps ARCNET controller with 2Kx8 on-board RAM

SMSC -> Microchip 

malfunctioning, the topology is set up incorrectly,

there is noise on the network, or a reconfiguration

is occurring.

The software should periodically place values in

the Tentative ID Register and monitor the New

Next ID bit to maintain an updated network map.

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.

OSCILLATOR

The COM20022 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 COM20022 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$ pull-up resistor is required on XTAL1, while

XTAL2 should be left unconnected.

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.

58

Share Link: