DS125BR800 Datasheet, PDF (12/59 Pages) Texas Instruments – Low-Power 12.5-Gbps 8-Channel Repeater With Input Equalization and Output De-Emphasis

DS125BR800
SNLS426E – AUGUST 2012 – REVISED JANUARY 2015
8 Detailed Description
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8.1 Overview
The DS125BR800 compensates for lossy printed circuit board backplanes and balanced cables.
The DS125BR800 operates in 3 modes: Pin Control Mode (ENSMB = 0), SMBus Slave Mode (ENSMB = 1) and
SMBus Master Mode (ENSMB = float) to load register information from external EEPROM; refer to SMBUS
Master Mode for additional information.
8.2 Functional Block Diagram
VDD
Auto / M anual
RXDET
RATE
DET
VOD/DeEMPHASIS
DEMA/B
CONTROL
SMBus
INx_n+
EQ
INx_n-
EQA/B
IDLE
DET
OUTBUF OUTx_n+
OUTx_n-
TX Idle Enable
SMBus
SMBus
Figure 8. Block Diagram - Detail View Of Channel (1 of 8)
8.3 Feature Description
8.3.1 4-level Input Configuration Guidelines
The 4-level input pins use a resistor divider to help set the 4 valid levels and provide a wider range of control
settings when ENSMB=0. There is an internal 30-kΩ pull-up and a 60-kΩ pulldown connected to the package
pin. These resistors, together with the external resistor connection combine to achieve the desired voltage level.
Using the 1-kΩ pull-up, 1-kΩ pull-down, no connect, and 20-kΩ pull-down provide the optimal voltage levels for
each of the four input states.
Table 1. 4-Level Input Voltage
LEVEL
SETTING
3.3-V MODE
2.5-V MODE
0
Tie 1 kΩ to GND
0.10 V
0.08 V
R
Tie 20 kΩ to GND
Float
Float (leave pin open)
1
• Level 1 - 2 = 0.2 × V_IN or V_DD
• Level 2 - 3 = 0.5 × V_IN or V_DD
• Level 3 - 4 = 0.8 × V_IN or V_DD
Tie 1 kΩ to V_DD
1/3 x V_DD
2/3 x V_DD
V_IN - 0.05 V
1/3 x V_DD
2/3 x V_DD
V_IN - 0.04 V
In order to minimize the startup current associated with the integrated 2.5-V regulator the 1 kΩ pullup and
pulldown resistors are recommended. If several 4-level inputs require the same setting, it is possible to combine
two or more 1-kΩ resistors into a single lower value resistor. As an example; combining two inputs with a single
500-Ω resistor is a good way to save board space.
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