LMH0395 Datasheet, PDF (13/21 Pages) Texas Instruments – 3G HD/SD SDI Dual Output Low Power Extended Reach Adaptive Cable Equalizer

In the second transaction, three dummy reads (each consist-
ing of 16 “1”s) are shifted in, and the read data from Device 2
(with value 0x88) appears on MISO in the 25th through 32nd
clock cycles.
SPI Daisy-Chain Length Detection
A useful operation for the host may be to detect the length of
the daisy-chain. This is a simple matter of shifting in a series
of dummy reads with a known data value (such as 0x5A). For
an SPI daisy-chain of N LMH0395 devices, the known data
value will appear on the host's MISO pin after N+1 writes.
Assuming a daisy-chain of three LMH0395 devices, the result
of this operation is shown in Figure 11.
FIGURE 11. SPI Daisy-Chain Length Detection
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OUTPUT DRIVER ADJUSTMENTS AND DE-EMPHASIS
SETTING
The output driver swing (amplitude), offset voltage (common
mode voltage), and de-emphasis level are adjustable via SPI
register 01h. The output driver to control with SPI register 01h
(either output driver 0 or output driver 1) can be selected via
bit 2 of SPI register 00h.
The output swing is adjustable via bits [7:6] of SPI register
01h. The default value for these register bits is “10” for a peak
to peak differential output voltage of 700 mV_P-P. The output
swing can be set for 400 mV_P-P, 600 mV_P-P , 700 mV_P-P, or
800 mV_P-P.
The offset voltage is adjustable via bits [5:4] of SPI register
01h. The default value for these register bits is “10” for an
output offset of 1.2V. The output common mode voltage may
be adjusted in 200 mV increments, from 0.8V to 1.2V. It can
be set to “11” for the maximum offset voltage. At this maxi-
mum offset voltage setting, the outputs are referenced to the
positive supply and the offset voltage is around 1.35V.
The output de-emphasis is turned on or off by bit 3 of SPI
register 01h, and the de-emphasis level is set by bits [2:1] of
SPI register 01h. The output de-emphasis level may be set
for 0 dB (for driving up to 10” FR4), 3 dB (for driving 10-20”
FR4), 5 dB (for driving 20-30” FR4), or 7 dB (for driving 30-40”
FR4).
LAUNCH AMPLITUDE OPTIMIZATION
The LMH0395 can compensate for attenuation of the input
signal prior to the equalizer. This compensation is useful for
applications with a passive splitter at the equalizer input or a
non-ideal input termination network, and is controlled by SPI
register 02h.
Bit 7 of SPI register 02h is used for the launch amplitude set-
ting. At the default setting of “0”, the LMH0395 operates
normally and expects a launch amplitude of 800 mV_P-P. Bit 7
may be set to “1” to optimize the LMH0395 for input signals
with 6 dB of attenuation (400 mV_P-P).
CABLE LENGTH INDICATOR (CLI)
The cable length indicator (CLI) provides an indication of the
length of the cable attached to input. CLI is accessible via bits
[7:0] of SPI register 06h. The 8-bit setting ranges in decimal
value from 0 to 247 (“00000000” to “11110111” binary), cor-
responding to 0 to 400m of Belden 1694A cable. For 3G and
HD input, CLI is 1.25m per step. For SD input, CLI is 1.25m
per step, less 20m, from 0 to 191 decimal, and 3.5m per step
from 192 to 247 decimal.
To calculate the Belden 1694A cable length (in meters) from
the CLI decimal value for 3G or HD input:
To calculate the Belden 1694A cable length (in meters) from
the CLI decimal value for SD input:
Figure 12 shows typical CLI values vs. Belden 1694A cable
length. CLI is valid for Belden 1694A cable lengths of 0-200m
at 2.97 Gbps, 0-220m at 1.485 Gbps, and 0-400m at 270
Mbps. Note: Given the continuous adaptive nature of the
equalizer, this setting changes by some steps constantly.
250
225
200
175
150
SD
125
100
75
50
3G/HD
25
0
0 50 100 150 200 250 300 350 400
BELDEN 1694A CABLE LENGTH (m)
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FIGURE 12. CLI vs. Belden 1694A Cable Length
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