SML2108
PRELIMINARY
the data is first placed in the volatile register. At the to normalize overall module operation.
conclusion of the write command an internal nonvolatile
writesequenceinitiatesthestorageofthevolatilecontents
into the NV register.
Althoughthememorymaynormallybereadandwrittenas
a standard memory, a security feature exists in the con-
figuration settings that will prevent any external access to
Notethatwhenmodifyingthe10-BitDACoutput,themean the array. Additionally, if the auto-monitor feature is not
power control loop will become temporarily disrupted. It used, then the modulation output current may be pro-
may be several milliseconds before the bias current has grammed to a fixed value, and the array may be used as
settled to its steady state value. Until then its value will be a standard memory to store device settings, board identi-
undefined.
fication values, production dates, etc.
Modulation Current — Auto-Monitor Control
8-Bit Current Output D/A
The laser bias current, which relates directly to laser The 8-Bit D/A defines the modulation output current.
temperature, can be monitored using an on-board, cur- Associated with this DAC are an 8-Bit volatile register and
rent-sensingA/Dconverter. Intheauto-monitormodethe an 8-Bit nonvolatile (NV) register. The content of the
8-Bit output of the converter is used as an address to the volatile register determines the DAC output current. The
EEPROMlookuptable. Thesubsequent8-Bitdataoutput DAC output current is given by the following relation:
from the lookup table becomes the input for the compen-
sation DAC. The 8-Bit compensation DAC output is a
X
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
3.
OC =
×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.
The lookup table provides an arbitrary mapping from bias
currenttomodulationcurrent. TheinputrangetotheADC
may be scaled and/or offset to provide maximum resolu-
tionwithintheappropriateconversionspace. Thesample
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.
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
immediatelyattheendofthewritecommand. Readingthe
volatileregisterhasnoeffectontheDACoutput. Reading
orwritingthevolatileregisterhasnoeffectonthecontents
of the nonvolatile register.
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.
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
thevolatileregister,exceptwordaddress6isusedinstead
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
writesequenceinitiatesthestorageofthevolatilecontents
into the NV register.
In the auto-monitor mode the content of the array repre-
sentsthetransferfunctionbetweentheA/Doutputandthe
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
useofalookuptableallowseachdevicetobecustomized
SUMMIT MICROELECTRONICS, Inc.
2053 2.2 11/07/00
9
SUMMIT [ SUMMIT MICROELECTRONICS, INC. ]
