Ds1862 xfp laser control and digital diagnostic ic, Applications information – Rainbow Electronics DS1862 User Manual
Page 40
DS1862
XFP Laser Control and Digital Diagnostic IC
40
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CRC-8 includes and requires the explicit starting mem-
ory address to be included as the second transferred
byte. Next, the master transfers the data as the DS1862
acknowledges. Only 4 bytes can be sequentially written
during one transmission while using PEC. After the
master writes the intended number of bytes, the CRC-8
value should be transmitted. Following the CRC-8 byte
the master should transmit the CAB byte (CRC Add-on
Byte). At this point, the DS1862 sends an ACK if the
CRC-8 matches its internal calculated value or a NACK
if not. Finally the master should end the communication
and send a STOP. See Figure 16 for a graphical repre-
sentation. The CRC-8 is calculated starting with the
MSB of the memory address pointer, number of bytes
to be written, and the written data. The master can then
poll the last ACK or NACK for successful transfer of
written data.
For more information on I
2
C PEC communications,
please refer to the XFP and/or SMBus 2.0 standard.
Applications Information
Calibrating APC and Extinction Ratio
Before calibrating, the APC register should be set to a
low value to ensure the laser’s maximum power level is
not exceeded before the power level is calibrated.
Additionally, the ER should be set to a minimum value
to ensure that a data test pattern does not cause the
laser to shut off. Once the APC and ER registers are at
minimal values, enable a data pattern and calibrate the
average power level.
Calibrating the Average Power Level
While sending data through the laser diode, increase
the value in the APC register until the light output
matches the desired average power level. The average
power level is the arithmetic average of the ‘1’ and ‘0’
power levels.
X
X
X
X
X
X
X
X
1
0
1
0
0
0
0
0
1
0
1
0
0
0
0
0
1
0
1
0
0
0
0
0
COMMUNICATIONS KEY
WRITE UP TO A 4-BYTE PAGE WITH A SINGLE TRANSACTION USING PEC
READ 1–128 BYTES WITH A DUMMY WRITE CYCLE TO SET THE ADDRESS COUNTER
8-BIT ADDRESS OR DATA
WHITE BOXES INDICATE THE MASTER IS
CONTROLLING SDA
THE FIRST BYTE SENT AFTER A START CONDITION IS
ALWAYS THE SLAVE ADDRESS FOLLOWED BY THE
READ/WRITE BIT.
SHADED BOXES INDICATE THE SLAVE IS
CONTROLLING SDA
START
ACK
NOT
ACK
S
S
S
A
A
A
A
A
A
A
SR
A
A
N
P
A
A
CRC-8 VALUE
DATA
DATA
DATA
CRC-8 VALUE
DATA
DATA
MEMORY ADDRESS
A
NUMBER OF BYTES
NUMBER OF BYTES
MEMORY ADDRESS
DATA
A
A
A
P
N
SR
STOP
REPEATED
START
NOTE:
ALL BYTES ARE SENT MOST SIGNIFICANT BIT FIRST.
A (IF CRC-8 IS CORRECT)
P
DATA
Figure 16. I
2
C PEC Communications Examples