DS1864 datasheet(16/72 Pages) DALLAS

16 / 72 page

SFP Laser Controller and

Diagnostic IC

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Detailed Description

The DS1864 manages all system monitoring functions

in a fiber-optic data transceiver module in accordance

with SFF-8472 MSA. The IC communicates with a host

system through a I2C bus, and can be programmed

with a unique I2C address.

The IC offers temperature-controlled lookup tables for

its two current-sink DACs. Monitoring and calibration

functions for supply voltage, temperature and three

analog signals are available, as well as programmable

alarm and warning flags for these signals which can be

used to trigger interrupts based on user-specified limits.

The IC also possesses laser shutdown (eye safety) fea-

tures such as programmable fast-trip alarms and inter-

rupts, in addition to signals such as FETG for laser

safety disconnect.

The memory is protected by a customizable two-layer

password scheme. Furthermore, the memory layout

can be configured to be compatible with the

DS1852/DS1856 or the DS1859.

An overview of the DS1864’s functions is shown in the

block diagram in Figure 11. Additional DS1864 func-

tions are shown in Figures 12 and 13.

Control Features

The DS1864 contains two current-sink DACs, DAC0 and

DAC1. Normally, each DAC is controlled by a tempera-

ture-indexed lookup table (LUT), which can change the

DAC settings based on the temperature measured by

the internal temperature sensor. However, each DAC

can also be manually programmed by the user.

DAC0 and DAC1

The current-sink DACs are linear and have two user-

selectable ranges, 1.5mA and 0.5mA. The range is

selected by the DAC0R and DAC1R bits located in

address 88h in Table 04h (Table 01h in DS1859 config-

uration). The 1.5mA range is selected when the corre-

sponding bit is set to a 1, and the 0.5mA range is

selected when the corresponding bit is set to a 0. The

temperature-indexed LUT for each DAC determines the

value to be loaded in to the DAC0 and DAC1 registers

(bytes 82h and 83h respectively in Table 04h (Table 01h

in DS1859 configuration)). The DACs can be disabled

(placed in a high-impedance mode) by pulling the TX-D

pin high. The TXDC control bit (Lower Memory Register,

byte 6Eh, bit 6) can also be used to disable the DAC

outputs by placing them in a high-impedance state.

To determine the DAC position to produce a desired

current, the following equation can be used:

Update bits are provided to indicate when an A/D con-

version has completed for each monitored value. These

bits are located in Lower Memory, byte 77h.

DAC Lookup Table (LUT) Operation

The current-sink DAC settings are determined by tem-

perature-controlled Lookup Tables (LUTs). The LUTs

are located in Table 02h for DAC0 and Table 03h for

DAC1. The lookup tables are 72 bytes each and allow

the biasing to be adjusted every 2°C between -40°C

and +102°C. Temperatures less than -40°C or greater

than +102°C use the -40°C or +102°C values, respec-

tively. The values programmed into the LUTs are 8-bit

unsigned values that represent the desired DAC setting

for each 2°C temperature window. The LUTs have 1°C

hysteresis (see Figure 14) to prevent the DAC’s setting

from chattering in the event the temperature remains

near a LUT switching point. Table 1 shows which regis-

ter corresponds to which temperature in the LUTs.

Figure 14 shows how the LUT chooses which memory

location to use for the DACs depending on the temper-

ature read from the internal temperature sensor.

The Temperature Index Byte (address 81h, Table 04h

(Table 01h in DS1859 configuration)) is automatically

calculated following each temperature conversion and

points to the corresponding location in the LUTs for the

DESIRED POSITION

DESIRED CURRENT

FULL SCALE CURRENT

⎛

⎝⎜

⎞

⎠⎟

× 255

9Ah

99h

98h

97h

96h

95h

TEMPERATURE (

°C)

INCREASING

TEMPERATURE

DECREASING

TEMPERATURE

Figure 14. LUT Hysteresis

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