DS125BR800A Datasheet, PDF (11/53 Pages) Texas Instruments – Low-Power 12.5-Gbps 8-Channel (Unidirectional) Repeater With Input Equalization

DS125BR800A
www.ti.com
SNLS467 – NOVEMBER 2013
FUNCTIONAL DESCRIPTION
The DS125BR800A compensates for lossy FR-4 printed circuit board backplanes and balanced cables. The
DS125BR800A 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; please refer to
SMBUS Master Mode for additional information.
Pin Control Mode:
When in pin mode (ENSMB = 0), equalization and de-emphasis can be selected via pin for each side
independently. When de-emphasis is asserted VOD is automatically adjusted per Table 4. For PCIe applications,
the RXDET pins provides automatic and manual control for input termination (50Ω or >50KΩ). MODE setting is
also pin controllable with pin selections (Gen 1/2, auto detect and SAS-3 / PCIe Gen 3). The receiver electrical
idle detect threshold is also adjustable via the SD_TH pin.
SMBUS Mode:
When in SMBus mode (ENSMB = 1), the VOD (output amplitude), equalization, de-emphasis, and termination
disable features are all programmable on a individual lane basis, instead of grouped by A or B as in the pin mode
case. Upon assertion of ENSMB, the EQx and DEMx functions revert to register control immediately. The EQx
and DEMx pins are converted to AD0-AD3 SMBus address inputs. The other external control pins (MODE,
RXDET and SD_TH) remain active unless their respective registers are written to and the appropriate override bit
is set, in which case they are ignored until ENSMB is driven low (pin mode). On power-up and when ENSMB is
driven low all registers are reset to their default state. If PWDN is asserted while ENSMB is high, the registers
retain their current state.
Equalization settings accessible via the pin controls were chosen to meet the needs of most PCIe applications. If
additional fine tuning or adjustment is needed, additional equalization settings can be accessed via the SMBus
registers. Each input has a total of 256 possible equalization settings. The tables show the 16 setting when the
device is in pin mode. When using SMBus mode, the equalization, VOD and de-Emphasis levels are set by
registers.
The 4-Level input pins utilize 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 30K pull-up and a 60K pull-down connected to the package pin.
These resistors, together with the external resistor connection combine to achieve the desired voltage level.
Using the 1K pull-up, 1K pull-down, no connect, and 20K pull-down provide the optimal voltage levels for each of
the four input states.
Table 2. 4-Level Control Pin Settings
Level
0
R
Float
1
Setting
Tie 1kΩ to GND
Tie 20kΩ to GND
Float (leave pin open)
Tie 1kΩ to V_DD
3.3V Mode
0.10 V
1/3 x V_IN
2/3 x V_IN
V_IN - 0.05 V
2.5V Mode
0.08 V
1/3 x V_DD
2/3 x V_DD
V_DD - 0.04 V
Typical 4-Level Input Thresholds
• 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
In order to minimize the startup current associated with the integrated 2.5V regulator the 1K pull-up / pull-down
resistors are recommended. If several 4 level inputs require the same setting, it is possible to combine two or
more 1K resistors into a single lower value resistor. As an example; combining two inputs with a single 500 Ohm
resistor is a good way to save board space.
Copyright © 2013 , Texas Instruments Incorporated
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