Datasheet

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DP83849I
3.3 10 Mb Serial Network Interface (SNI)
The DP83849I incorporates a 10 Mb Serial Network Inter-
face (SNI) which allows a simple serial data interface for 10
Mb only devices. This is also referred to as a 7-wire inter
-
face. While there is no defined standard for this interface, it
is based on early 10 Mb physical layer devices. Data is
clocked serially at 10 MHz using separate transmit and
receive paths. The following pins are used in SNI mode:
—TX_CLK
—TX_EN
—TXD[0]
—RX_CLK
—RXD[0]
— CRS
—COL
3.4 Single Clock MII Mode
Single Clock MII (SCMII) Mode allows MII operation using
a single 25MHz reference clock. Normal MII Mode requires
three clocks, a reference clock for physical layer functions,
a Transmit MII clock, and a Receive MII clock. Similar to
RMII mode, Single Clock MII mode requires only the refer
-
ence clock. In addition to reducing the number of pins
required, this mode allows the attached MAC device to use
only the reference clock domain. Since the DP83849I has
two ports, this actually reduces the number of clocks from 6
to 1. A/C Timing requirements for SCMII operation are
similar to the RMII timing requirements.
For 10Mb operation, as in RMII mode, data is sampled and
driven every 10 clocks since the reference clock is at 10x
the data rate.
Separate control bits allow enabling the Transmit and
Receive Single Clock modes separately, allowing just
transmit or receive to operate in this mode. Control of Sin
-
gle Clock MII mode is through the RBR register.
Single Clock MII mode incorporates the use of the RMII
elasticity buffer, which is required to tolerate potential fre-
quency differences between the 25MHz reference clock
and the recovered receive clock. Settings for the Elasticity
Buffer for SCMII mode are detailed in the following table.
Table 5. Supported SCMII packet sizes at +/-50ppm frequency accuracy
Start Threshold
RBR[1:0]
Latency Tolerance Recommended Packet Size
at +/- 50ppm
100Mb 10Mb 100Mb 10Mb
01 (default) 4 bits 8 bits 4,000 bytes 9,600 bytes
10 4 bits 8 bits 4,000 bytes 9,600 bytes
11 12 bits 8 bits 9.600 bytes 9,600 bytes
00 12 bits 8 bits 9,600 bytes 9,600 bytes