DP83953 datasheet(43/90 Pages) NSC

43 / 90 page

5.0 HUB MANAGEMENT SUPPORT (Continued)

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Collision Bit Timer

The Collision Timer counts, in bit times, the time between

the start of repetition of the packet and the detection of the

packet's first collision. When a collision occurs, the Colli-

sion counter increments as the packet repeats and freezes.

The value in the counter is only valid when the collision bit

"COL" in [PSR(1)] is set.

Repeat Byte Counter

The Repeat Byte Counter is a 16 bit counter that can per-

form two functions. In cases where the transmitted packet

possesses an SFD, the byte counter counts the number of

received bytes after the SFD field. Alternatively, if no SFD

is repeated, the counter reflects the length of the packet

(counted in bytes) starting at the beginning of the preamble

field. When performing the latter function, the counter is

shortened to 7 bits when MPS =0 in the GSR register.

Thus, the maximum count value is 127 bytes. The counter

is shortened to 11 bits when MPS =1 in the GSR register.

In this configuration, the maximum received byte count

changes to 2048 bytes. The mode of counting is indicated

by the "NSFD" bit in [PSR(2)]. In order to check if the

received packet was genuinely a Non-SFD packet, the sta-

tus of the COL bit should be checked. During collisions

SFD fields may be lost or created, Management software

should be robust to this kind of behavior.

Inter Frame Gap (IFG) Bit Timer

The IFG counter counts, in bit times, the period in between

repeater transmissions. The IFG counter increments when-

ever the RIC2A is not transmitting a packet. If the IFG is

long, i.e., greater than 255 bit times, the counter holds this

value. Thus a count value equal to 255 should be inter-

preted as 255 or more bit times.

5.4 Description of Hardware Connection for Man-

agement Interface

The RIC2A has been designed so that it may be connected

to the management bus directly or to external bus trans-

ceivers. External bus transceivers are advantageous in

larger repeaters because system backplanes are often

heavily loaded beyond the drive capabilities of the on-chip

bus drivers.

The unidirectional nature of information transfer on the

MCRS, MRXD and MRXC signals, means a single open

drain output pin is adequate for each of these signals. The

Management Enable (MEN) RIC2A output pin performs the

function of a drive enable for an external bus transceiver if

one is required.

In common with the Inter-RIC bus signals (ACTN, ANYXN,

COLN and IRE) the MCRS active level asserted by the

MCRS output is determined by the state of the BINV Mode

Load configuration bit.

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