SML2108F datasheet(9/21 Pages) SUMMIT

9 / 21 page

2053 2.2 11/07/00

SML2108

SUMMIT MICROELECTRONICS, Inc.

PRELIMINARY

the data is first placed in the volatile register. At the

conclusion of the write command an internal nonvolatile

write sequence initiates the storage of the volatile contents

into the NV register.

Note that when modifying the 10-Bit DAC output, the mean

power control loop will become temporarily disrupted. It

may be several milliseconds before the bias current has

settled to its steady state value. Until then its value will be

undefined.

Modulation Current — Auto-Monitor Control

The laser bias current, which relates directly to laser

temperature, can be monitored using an on-board, cur-

rent-sensing A/D converter. In the auto-monitor mode the

8-Bit output of the converter is used as an address to the

EEPROM lookup table. The subsequent 8-Bit data output

from the lookup table becomes the input for the compen-

sation DAC. The 8-Bit compensation DAC output is a

current in the range of 0 to 100mA and is used to control

the modulation current MODP and MODN. The output

block of the modulation current control is shown in Figure

The lookup table provides an arbitrary mapping from bias

current to modulation current. The input range to the ADC

may be scaled and/or offset to provide maximum resolu-

tion within the appropriate conversion space. The sample

interval is programmable from 10µs to 1s. Refer to the

ADC section for further details about configuring the A/D.

The interface is used to program the configuration regis-

ters as well as lookup table values.

Lookup Table

A 2k-Bit (256 x 8) memory array of on-board EEPROM

comprises the internal lookup table. This array is ac-

cessed via the 2-wire serial interface using a slave ad-

dress of 1010BIN. (Note: 1010BIN is the default, however

this may be set to 1110BIN, depending upon the contents

of Configuration Register 2.) Refer to the Bus Interface

section for details on programming and reading data from

the device.

In the auto-monitor mode the content of the array repre-

sents the transfer function between the A/D output and the

final value of modulation current. Using a lookup table to

implement this function allows arbitrary functions, and

even nonlinear relations, to be easily realized. Also, the

use of a lookup table allows each device to be customized

to normalize overall module operation.

Although the memory may normally be read and written as

a standard memory, a security feature exists in the con-

figuration settings that will prevent any external access to

the array. Additionally, if the auto-monitor feature is not

used, then the modulation output current may be pro-

grammed to a fixed value, and the array may be used as

a standard memory to store device settings, board identi-

fication values, production dates,

etc.

8-Bit Current Output D/A

The 8-Bit D/A defines the modulation output current.

Associated with this DAC are an 8-Bit volatile register and

an 8-Bit nonvolatile (NV) register. The content of the

volatile register determines the DAC output current. The

DAC output current is given by the following relation:

100mA

256

=×

where X = the 8-Bit data stored in the volatile register. On

device power-up the volatile register may be loaded with

all zeroes or it may be loaded from the contents of the 8-

Bit nonvolatile register.

Access to the 8-Bit volatile register is obtained via the 2-

wire interface at slave address 1001BIN, word address 4.

Refer to Figures 8 and 11 for details on programming and

reading data from the 8-Bit register. When writing to the

volatile register, the new DAC output will become valid

immediately at the end of the write command. Reading the

volatile register has no effect on the DAC output. Reading

or writing the volatile register has no effect on the contents

of the nonvolatile register.

The 8-Bit NV register can only be accessed indirectly

through the volatile register. The command sequence to

communicate with the NV register is the same as that of

the volatile register, except word address 6 is used instead

of 4.

When reading the NV register the data is first

transferred into the volatile register where it may be

accessed by the serial interface. Note that upon this

transfer the DAC output will change immediately to reflect

the new data. Similarly, when writing to the NV register,

the data is first placed in the volatile register. At the

conclusion of the write command, an internal nonvolatile

write sequence initiates the storage of the volatile contents

into the NV register.

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