COM20020ILJP datasheet(47/72 Pages) MICROCHIP

47 / 72 page

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

Datasheet

SMSC COM20020I Rev D

Page 47

Revision 12-05-06

DATASHEET

6.6

Reset Details

6.6.1

Internal Reset Logic

The COM20020ID includes special reset circuitry to guarantee smooth operation during reset. Special care

is taken to assure proper operation in a variety of systems and modes of operation. The COM20020ID

contains digital filter circuitry and a Schmitt Trigger on the nRESET signal to reject glitches in order to

ensure fault-free operation.

The COM20020ID supports two reset options; software and hardware reset. A software reset is generated

when a logic "1" is written to bit 7 of the Configuration Register. The device remains in reset as long as

this bit is set. The software reset does not affect the microcontroller interface modes determined after

hardware reset, nor does it affect the contents of the Address Pointer Registers, the Configuration

input. The minimum reset pulse width is 5TXTL. This pulse width is used by the internal digital filter, which

filters short glitches to allow only valid resets to occur.

Upon reset, the transmitter portion of the device is disabled and the internal registers assume those states

outlined in the Internal Registers section. After the nRESET signal is removed the user may write to the

internal registers. Since writing a non-zero value to the Node ID Register wakes up the COM20020ID

core, the Setup1 Register should be written before the Node ID Register. Once the Node ID Register is

written to, the COM20020ID reads the value and executes two write cycles to the RAM buffer. Address 0 is

written with the data D1H and address 1 is written with the Node ID. The data pattern D1H was chosen

arbitrarily, and is meant to provide assurance of proper microsequencer operation.

6.7

Initialization Sequence

6.7.1

Bus Determination

Writing to and reading from an odd address location from the COM20020ID's address space causes the

COM20020ID to determine the appropriate bus interface.

When the COM20020ID is powered on the

internal registers may be written to. Since writing a non-zero value to the Node ID Register wakes up the

core, the Setup1 Register should be written to before the Node ID Register. Until a non-zero value is

placed into the NID Register, no microcode is executed, no tokens are passed by this node, and no

reconfigurations are generated by this node. Once a non-zero value is placed in the register, the core

wakes up, but the node will not attempt to join the network until the TX Enable bit of the Configuration

Before setting the TX Enable bit, the software may make some determinations. The software may first

observe the Receive Activity and the Token Seen bits of the Diagnostic Status Register to verify the health

of the receiver and the network.

Next, the uniqueness of the Node ID value placed in the Node ID Register is determined. The TX Enable

bit should still be a logic "0" until it is ensured that the Node ID is unique. If this node ID already exists, the

Duplicate ID bit of the Diagnostic Status Register is set after a maximum of 420mS (or 840mS if the ET1

and ET2 bits are other than 1,1). To determine if another node on the network already has this ID, the

COM20020ID compares the value in the Node ID Register with the DID's of the token, and determines

whether there is a response to it. Once the Diagnostic Status Register is read, the DUPID bit is cleared.

The user may then attempt a new ID value, wait 420mS before checking the Duplicate ID bit, and repeat

the process until a unique Node ID is found. At this point, the TX Enable bit may be set to allow the node

to join the network. Once the node joins the network, a reconfiguration occurs, as usual, thus setting the

MYRECON bit of the Diagnostic Status Register.

COM20020ILJP Datasheet(PDF) 47 Page - Microchip Technology

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