Datasheet

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DP83849C
7.3.7 Variance Control Register (VAR_CTRL), Page 2, address 1Ah
The Variance Control and Data Registers provide control and status for the Cable Signal Quality Estimation function.
The Cable Signal Quality Estimation allows a simple method of determining an approximate Signal-to-Noise Ratio for the
100Mb receiver. This register contains the programmable controls and status bits for the variance computation, which
can be used to make a simple Signal-to-Noise Ratio estimation.
7.3.8 Variance Data Register (VAR_DATA), Page 2, address 1Bh
This register contains the 32-bit Variance Sum. The contents of the data are valid only when VAR_RDY is asserted in the
VAR_CTRL register. Upon detection of VAR_RDY asserted, software should set the VAR_FREEZE bit in the VAR_CTRL
register to prevent loading of a new value into the VAR_DATA register. Since the Variance Data value is 32-bits, two
reads of this register are required to get the full value.
Table 49. Variance Control Register (VAR_CTRL), address 1Ah
Bit Bit Name Default Description
15 VAR_RDY
0, RO
Variance Data Ready Status:
Indicates new data is available in the Variance data register. This
bit will be automatically cleared after two consecutive reads ot
VAR_DATA.
14:4 RESERVED 0, RO RESERVED: Writes ignored, read as 0.
3 VAR_FREEZE
0, RW
Freeze Variance Registers:
Freeze VAR_DATA register.
This bit is ensures that VAR_DATA register is frozen for software
reads. This bit is automatically cleared after two consecutive reads
of VAR_DATA.
2:1 VAR_TIMER 0, RW Variance Computation Timer (in ms):
Selects the Variance computation timer period. After a new value
is written, computation is automatically restarted. New variance
register values are loaded after the timer elapses.
Var_Timer = 0 => 2 ms timer (default)
Var_Timer = 1 => 4 ms timer
Var_Timer = 2 => 6 ms timer
Var_Timer = 3 => 8 ms timer
Time units are actually 2
17
cycles of an 8ns clock, or 1.048576ms.
0 VAR_ENABLE
0, RW
Variance Enable:
Enable Variance computation. Off by default.
Table 50. Variance Data Register (VAR_DATA), address 1Bh
Bit Bit Name Default Description
15:0 VAR_DATA 0, RO Variance Data:
Two reads are required to return the full 32-bit Variance Sum value.
Following setting the VAR_FREEZE control, the first read of this
register will return the low 16 bits of the Variance data. A second
read will return the high 16 bits of Variance data.