KSZ8841-PMQL(2006) Просмотр технического описания (PDF) - Micrel
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KSZ8841-PMQL Datasheet PDF : 74 Pages
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Micrel, Inc.
KSZ8841-PMQL
On the transmit side, the KSZ8841-PMQL has intelligent and efficient ways to determine when to invoke flow control.
The flow control is based on availability of the system resources.
The KSZ8841-PMQL issues a flow control frame (XOFF, or transmitter off), containing the maximum pause time
defined in IEEE standard 802.3x. Once the resource is freed up, the KSZ8841-PMQL sends out another flow control
frame (XON, or transmitter on) with zero pause time to turn off the flow control (turn on transmission to the port). A
hysteresis feature is provided to prevent the flow control mechanism from being constantly activated and deactivated.
Half-Duplex Backpressure
A half-duplex backpressure option (non-IEEE 802.3 standards) is also provided. The activation and deactivation
conditions are the same as above in full-duplex mode. If backpressure is required, the KSZ8841-PMQL sends
preambles to defer the other stations' transmission (carrier sense deference).
To avoid jabber and excessive deference (as defined in the 802.3 standard), after a certain time, the KSZ8841-PMQL
discontinues the carrier sense and then raises it again quickly. This short silent time (no carrier sense) prevents other
stations from sending out packets thus keeping other stations in a carrier sense deferred state. If the port has packets
to send during a backpressure situation, the carrier sense type backpressure is interrupted and those packets are
transmitted instead. If there are no additional packets to send, carrier sense type backpressure is reactivated again until
chip resources free up. If a collision occurs, the binary exponential back-off algorithm is skipped and carrier sense is
generated immediately, thus reducing the chance of further colliding and maintaining carrier sense to prevent packet
reception.
The backpressure will take effect automatically in Auto-negotiation enable and half-duplex mode.
Clock Generator
The X1 and X2 pins are connected to a 25MHz crystal. X1 can also serve as the connector to the 3.3V 25MHz oscillator
(as described in the pin description).
EEPROM Interface
An external serial EEPROM with a standard microwire bus interface is used for non-volatile storage of information such
as the node address and subsystem ID.
As part of the initialization after system reset, the KSZ8841-PMQL reads the external EEPROM and places the data
into certain host-accessible registers if the EEEN pin is pulled-up, the KSZ8841-PMQL performs an automatic read of
the EEPROM word from 0x0 to 0x6 after the de-assertion of Reset. An EEPROM of 1KB(93C46) or 4KB(93C66) can be
used based on application.
The EEPROM read/write function can also be performed by software reading and writing to the EEPCR register.
The KSZ8841-PMQL EEPROM format is given in Table 2.
WORD
0x0
0x1
0x2
0x3
0x4
0x5
0x6
0x7-0x3F
15
87
0
Reserved
MAC Address Byte 2
MAC Address Byte 1
MAC Address Byte 4
MAC Address Byte 3
MAC Address Byte 6
MAC Address Byte 5
Subsystem ID
Subsystem Vendor ID
ConfigParam
Not used by KSZ8841-PMQL (available for user to use)
Table 2. KSZ8841-PMQL EEPROM Format
June 2006
21
M9999-061206-1.2
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