DS1864 datasheet(18/72 Pages) DALLAS

18 / 72 page

SFP Laser Controller and

Diagnostic IC

____________________________________________________________________

Internal calibration applies to measured values acquired

by the ADC, and does not apply to the fast alarms. If inter-

nal calibration is desired, each analog channel requires

that registers 8Eh through AFh in Table 04h (Table 01h in

DS1859 configuration) are loaded with the appropriate val-

ues to calibrate for gain and offset. Every gain and offset

are independently capable of converting input variables

into a digital output range spanning 0000h to FFFFh.

The last adjustment is made by using right-shifting.

Right-shifting registers are located in registers A2h

through ABh and AEh to AFh, and store a 3-bit value

used to shift each MON value from 0 to 7 spaces to the

right. The effect of this is to make better use of the ADC

range and increase the accuracy of the readings. Right-

shifting is the last function performed on the MON sig-

nal before the digital value is sent to the MON register.

Temperature Monitor Operation

The internal temperature monitor values are stored in

16-bit 2’s complement format, and located in memory

addresses 60h and 61h of the Lower Memory. The tem-

perature conversions are updated every tFRAME, and

do not occur during an active read or write to memory.

The factory default calibration values for the tempera-

ture monitor are shown in Table 2.

To convert the 2s complement register value to the tem-

perature it represents, first convert the 2-byte hexadeci-

mal value to a decimal value as if it is an unsigned value,

then divide the result by 256. Finally, subtract 256 if the

result of the division is greater than or equal to +128.

Example converted values are shown in Table 3 below.

The offset of the temperature sensor can be adjusted

using the internal calibration registers to account for

differences between the ambient temperature at the

location of the DS1864 and the temperature of the

device it is biasing. When offsets are applied to the

temperature measurement, the value converted is off-

set by a fixed value from the DS1864’s ambient temper-

ature. For more information, see the following

Temperature Monitor Offset Calibration section.

Temperature Monitor Offset Calibration

The DS1864’s temperature sensor comes precalibrated

and requires no further adjustment by the customer for

proper operation. However, it is possible to characterize

a system and add a fixed offset to the DS1864’s temper-

ature reading so it is representative of another location’s

temperature. This is not required for biasing because

the temperature offset can be accounted for by adjust-

ing the data’s location in the LUTs, but this feature is

available for customers that see application benefits.

To change the temperature sensor’s offset: write the

temperature offset register to 0000h, measure the

source reference temperature (TREF, °C), and read the

temperature from the DS1864 (TDS1864, °C). Then, the

following formula can be used to calculate the value for

the temperature offset register.

Once the value is calculated, write it to the temperature

offset register.

Voltage Monitor Operation

In addition to monitoring temperature, the DS1864 mon-

itors VCC and the three MON inputs in a round-robin

fashion using its 13-bit A/D converter. The converted

values are stored in memory addresses 62h to 69h as

16-bit unsigned numbers with the ADC results left justi-

fied in the register. The round-robin update time is

specified by tFRAME in the analog voltage monitoring

characteristics.

The default factory-calibrated values for the voltage

monitors are shown in Table 4.

By using the internal gain and offset calibration regis-

ters the +FS and -FS signal values shown in Table 4

can be modified to meet customer needs. For more

information on calibration, see the following Voltage

Monitor Calibration section.

Note: ±FS voltages shown in Table 4 were calculated

assuming factory-programmed gain and offset values

in addition to right shifting set to 0.

TEMP OFFSET

XOR

REF

BITWISE

=× −

−

()

275

1864

()

SIGNAL

+FS

SIGNAL

+FS

(hex)

-FS

SIGNAL

-FS

(hex)

Temperature

+127.96875°C

7FF8

-128.00°C

8000

Table 2. Internal Temperature Monitor

Factory Default Calibration

Table 3. Temperature Conversion Values

MSB

(bin)

LSB

(bin)

TEMPERATURE

(°C)

01000000

00000000

01000000

00001111

64.059

01011111

00000000

11110110

00000000

-10

11011000

00000000

-40

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