Rainbow Electronics MAX3798 - Manual (Page 20)

MAX3798
3-Wire Digital Communication
The MAX3798 implements a proprietary 3-wire digital
interface. An external controller generates the clock. The
3-wire interface consists of an SDA bidirectional data
line, an SCL clock signal input, and a CSEL chip-select
input (active high). The external master initiates a data
transfer by asserting the CSEL pin. The master starts to
generate a clock signal after the CSEL pin has been set
to 1. All data transfers are most significant bit (MSB) first.
Protocol
Each operation consists of 16-bit transfers (15-bit
address/data, 1-bit RWN). The bus master generates
16 clock cycles to SCL. All operations transfer 8 bits to
the MAX3798. The RWN bit determines if the cycle is
read or write. See Table 4.
Register Addresses
The MAX3798 contains 17 registers available for pro-
gramming. Table 5 shows the registers and addresses.
Write Mode (RWN = 0)
The master generates 16 clock cycles at SCL in total.
The master outputs a total of 16 bits (MSB first) to the
SDA line at the falling edge of the clock. The master
closes the transmission by setting CSEL to 0. Figure 4
shows the interface timing.
Read Mode (RWN = 1)
The master generates 16 clock cycles at SCL in total.
The master outputs a total of 8 bits (MSB first) to the
SDA line at the falling edge of the clock. The SDA line is
released after the RWN bit has been transmitted. The
slave outputs 8 bits of data (MSB first) at the rising edge
of the clock. The master closes the transmission by set-
ting CSEL to 0. Figure 4 shows the interface timing.
Mode Control
Normal mode allows read-only instruction for all regis-
ters except MODINC and BIASINC. The MODINC and
BIASINC registers can be updated during normal
mode. Doing so speeds up the laser control update
through the 3-wire interface by a factor of two. The nor-
mal mode is the default mode.
Setup mode allows the master to write unrestricted data
into any register except the status (TXSTAT1, TXSTAT2,
and RXSTAT) registers. To enter the setup mode, the
MODECTRL register (address = H0x0E) must be set to
H0x12. After the MODECTRL register has been set to
H0x12, the next operation is unrestricted. The setup
mode is automatically exited after the next operation is
finished. This sequence must be repeated if further
unrestricted settings are necessary.
1.0625Gbps to 10.32Gbps, Integrated, Low-
Power SFP+ Limiting Amplifier and VCSEL Driver
20 ______________________________________________________________________________________
PIN NAME SHORT TO V CC SHORT TO GND OPEN
31 CAZ2 Normal (Note 1) Normal (Note 1) Normal (Note 1)
32
CAZ1
(V EER )
Disabled—Fault (external supply
shorted) (Note 2)
Normal (Note 3)—Redundant path Normal (Note 3)—Redundant path
Table 3. Circuit Response to Single-Point Faults (continued)
BIT
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Register Address RWN Data that is written or read.
Table 4. Digital Communication Word Structure
Note 1: Normal—Does not affect laser power.
Note 2: Supply-shorted current is assumed to be primarily on the circuit board (outside this device) and the main supply is
collapsed by the short.
Note 3: Normal in functionality, but performance could be affected.
Warning: Shorted to V
CC or shorted to ground on some pins can violate the Absolute Maximum Ratings.