Datasheet

DS1878
SFP+ Controller with Digital LDD Interface
18 ______________________________________________________________________________________
Enhanced RSSI Monitoring (Dual-Range
Functionality)
The device offers a feature to improve the accuracy
and range of MON3, which is most commonly used for
monitoring RSSI. Using a traditional input, the accuracy
of the RSSI measurements is increased at the cost of
reduced input signal swing. The device eliminates this
trade-off by offering “dual range” calibration on the
MON3 channel.
The dual-range calibration can operate in two modes:
crossover enabled and crossover disabled.
• Crossover Enabled: For systems with a nonlinear
relationship between the ADC input and desired ADC
result, the mode should be set to crossover enabled
(Figure 8). The RSSI measurement of an APD receiv-
er is one such application. Using the crossover
enabled mode allows a piecewise linear approxima-
tion of the nonlinear response of the APD’s gain fac-
tor. The crossover point is the point between fine and
coarse points. The ADC result transitions between the
fine and coarse ranges with no hysteresis. Right-shift-
ing, slope adjustment, and offset are configurable for
both the fine and coarse ranges. The XOVER FINE
register determines the maximum results returned by
fine ADC conversions, before right-shifting. The
XOVER COARSE register determines the minimum
results returned by coarse ADC conversions, before
right-shifting.
• Crossover Disabled: The crossover disabled mode
is intended for systems with a linear relationship
between the MON3 input and desired ADC result.
The ADC result transitions between the fine and
coarse ranges with hysteresis (Figure 9). In crossover
disabled mode, the thresholds between coarse and
fine mode are a function of the number of right-shifts
being used. With the use of right-shifting, the fine-
mode full scale is programmed to (1/2
nth
) of the
coarse-mode full scale. The device now auto ranges
to choose the range that gives the best resolution for
the measurement. Table 4 shows the threshold val-
ues for each possible number of right-shifts.
CROSSOVER POINT
RSSI RESULT
IDEAL RESPONSE
MON3 INPUT
Figure 8. RSSI with Crossover Enabled
Table 5. MON3 Configuration Registers
REGISTER FINE MODE COARSE MODE
GAIN 98h–99h, Table 02h 9Ch–9Dh, Table 02h
OFFSET A8h–A9h, Table 02h ACh–ADh, Table 02h
RIGHT-SHIFT
0
8Fh, Table 02h 8Fh, Table 02h
CNFGC
(RSSI_FC and
RSSI_FF Bits)
8Bh, Table 02h
UPDATE
(RSSIR Bit)
6Fh, Lower Memory
MON3 VALUE 68h–69h, Lower Memory
NUMBER OF
RIGHT-SHIFTS
FINE MODE
MAX (HEX)
COARSE MODE
MIN* (HEX)
0 FFF8 F000
1 7FFC 7800
2 3FFE 3C00
3 1FFF 1E00
4 0FFF 0F00
5 07FF 0780
6 03FF 03C0
7 01FF 01E0
Table 4. MON3 Hysteresis Threshold Values
* This is the minimum reported coarse-mode conversion.