DS1877 datasheet(14/65 Pages) MAXIM

14 / 65 page

Maxim Integrated

SFP Controller for Dual Rx Interface

DS1877

Monitors and Fault Detection

Monitors

Monitoring functions on the device include two QT com-

parators and four ADC channels. This monitoring com-

bined with the alarm enables (Table 01h/05h) determines

when/if the device triggers the FAULT and/or LOSOUT

outputs. All the monitoring levels and interrupt masks are

user programmable.

Two Quick-Trip Monitors and Alarms

Two quick-trip monitors are provided that monitor the

following:

1) Loss of signal 1 (LOS1 LO)

2) Loss of signal 2 (LOS2 LO)

The LOS_ LO QTs compare the RSSI_ input against its

threshold setting to determine if the present received

power is below the specification. The LOS_ LO QT can

be used to set the LOSOUT pin.

Four ADC Monitors and Alarms

The ADC monitors 4 channels that measure temperature

(internal temp sensor), VCC, RSSI1, and RSSI2 using an

analog multiplexer to measure them round-robin with a

single ADC (see the ADC Timing section). The 3V chan-

nels have a customer-programmable full-scale range,

and all channels have a customer-programmable offset

value that is factory programmed to a default value (see

Table 2). Additionally, RSSI1 and RSSI2 can right-shift

results by up to 7 bits before the results are compared

to alarm thresholds or read over the I2C bus. This allows

customers with specified ADC ranges to calibrate the

ADC full scale to a factor of 1/2n of their specified range

to measure small signals. The device can then right-shift

the results by n bits to maintain the bit weight of their

specification (see the Right-Shifting ADC Result section).

The ADC results (after right-shifting, if used) are com-

pared to the alarm and warning thresholds after each

conversion, and the corresponding alarms are set that

can be used to trigger the FAULT output. These ADC

thresholds are user programmable, as are the masking

registers that can be used to prevent the alarms from

triggering the FAULT output.

ADC Timing

There are four analog channels that are digitized in a

round-robin fashion in the order as shown in Figure 3.

The total time required to convert all 4 channels is tRR

(see the Analog Voltage Monitoring Characteristics for

details).

Right-Shifting ADC Result

If the weighting of the ADC digital reading must conform

to a predetermined full-scale (PFS) value defined by

a standard’s specification (e.g., SFF-8472), then right-

shifting can be used to adjust the PFS analog measure-

ment range while maintaining the weighting of the ADC

results. The device’s range is wide enough to cover all

requirements; when the maximum input value is P 1/2

the FS value, right-shifting can be used to obtain greater

accuracy. For instance, the maximum voltage might be

1/8 the specified PFS value, so only 1/8 of the converter’s

range is effective over this range. An alternative is to cali-

brate the ADC’s full-scale range to 1/8 the readable PFS

value and use a right-shift value of 3. With this implemen-

tation, the resolution of the measurement is increased by

Table 2. ADC Default Monitor Full-Scale Ranges

Figure 3. ADC Round-Robin Timing

TEMP

VCC

RSSI1

FINE

RSSI1

COARSE

RSSI2

FINE

RSSI2

COARSE

TEMP

ONE ROUND-ROBIN ADC CYCLE

tRR

NOTE: IF THE VCC LO ALARM IS ENABLED AT POWER-UP, THE ADC ROUND-ROBIN TIMING CYCLES BETWEEN TEMPERATURE AND VCC ONLY UNTIL VCC IS ABOVE THE VCC LO

ALARM THRESHOLD.

SIGNAL (UNITS)

+FS SIGNAL

+FS HEX

-FS SIGNAL

-FS HEX

Temperature (NC)

127.996

7FFF

-128

8000

VCC (V)

6.5528

FFF8

0000

RSSI1, RSSI2 (V)

2.4997

FFF8

0000

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