DS1877T+ Maxim Integrated Products, DS1877T+ Datasheet - Page 15

• Manufacturer Part Number: DS1877T+
• Description: IC CTLR/MON SFP 1-2CH 28TQFN
• Manufacturer: Maxim Integrated Products
• Type: SFP+ Controllerr

Specifications of DS1877T+

• Input Type: Logic
• Output Type: Logic
• Interface: I²C
• Current - Supply: 2.5mA
• Mounting Type: Surface Mount
• Package / Case: 28-WFQFN exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 90-1877T+000

Figure 4. RSSI1/RSSI2 Differential Input for High-Side RSSI
Table 3. RSSI1/RSSI2 Configuration Registers
a factor of 8, and because the result is digitally divided
by 8 by right-shifting, the bit weight of the measurement
still meets the standard’s specification (i.e., SFF-8472).
The right-shift operation on the ADC result is carried
out based on the contents of right-shift control regis-
ters (Table 02h, Registers 8Eh–8Fh) in EEPROM. Two
analog channels—RSSI1 and RSSI2—each have 3 bits
allocated to set the number of right-shifts. Up to seven
right-shift operations are allowed and are executed as a
part of every conversion before the results are compared
to the high and low alarm levels, or loaded into their
corresponding measurement registers (Lower Memory,
Registers 64h to 6Bh). This is true during the setup of
internal calibration as well as during subsequent data
conversions.
The device offers fully differential inputs for RSSI1 and
RSSI2. This enables high-side monitoring of RSSI, as
shown in Figure 4. It also reduces board complexity by
(XOVER1/XOVER2 FINE/COARSE)
(RSSI1/2 FINE/COARSE OFFSET)
(RSSI1/2 FINE/COARSE SCALE)
(RSSIn_FC and RSSIn_FF Bits)
RSSI VALUE (RSSI1/RSSI2
ROSA
(RSHIFT
RSSI1/RSSI2 Offset
RSSI1/RSSI2 Gain
Measurement)
FORCE RSSI
REGISTER
(RSSIR Bit)
Right-Shift
Crossover
R
UPDATE
V
CC
1
, RSHIFT
Differential RSSI1/RSSI2 Inputs
RSSI_P
RSSI_N
2
)
SFP Controller for Dual Rx Interface
DS1877
ADC
AEh–AFh/AAh–ABh,
8Eh–8Fh, Table 02h
9Eh–9Fh/9Ah–9Bh,
A6h–A7h/96h–97h,
FINE MODE
Table 02h
Table 02h
Table 02h
eliminating the need for a high-side differential amplifier
or a current mirror.
The device offers a feature to improve the accuracy and
range of RSSI1/RSSI2, which is most commonly used
for monitoring RSSI. Using a traditional input, the RSSI
measurement accuracy can be increased at the cost
of reduced input signal swing. The device eliminates
this trade-off by offering “dual-range” calibration on the
RSSI1/RSSI2 channels. The dual-range calibration can
operate in two modes: crossover enabled and crossover
disabled. Dual-range operation is enabled by default
(factory programmed in EEPROM). However, it can
easily be disabled by the RSSIn_FC and RSSIn_FF bits
(where n can be 1 or 2) in 8Dh, Table 02h.
Dual-range functionality consists of two ADC modes of
operation: fine mode and coarse mode. Each mode is
calibrated for a unique transfer function, hence the term,
dual range. Table 3 highlights the registers related to
RSSI1/RSSI2. Fine mode is calibrated using the gain,
offset, and right-shifting registers at locations shown in
Table 3, and is ideal for relatively small analog-input
voltages. Coarse mode is automatically switched to
when the input exceeds a threshold. Coarse mode is
calibrated using different gain and offset registers from
fine mode. The gain and offset registers for coarse mode
are also shown in Table 3. Additional information for
each of the registers can be found in the memory map
(Figure 14).
68h–69h, Lower Memory
6Fh, Lower Memory
8Dh, Table 02h
ACh–ADh/A8h–A9h,
8Eh–8Fh, Table 02h
A4h–A5h/94h–95Fh,
9Ch–9Dh/98h–99h,
Enhanced RSSI Monitoring
(Dual-Range Functionality)
COARSE MODE
Table 02h
Table 02h
Table 02h
15