9.2.3 Detailed Design Procedure

The design procedure for backplane/mid-plane applications is as follows:

1. Determine the total number of channels on the board which require a DS250DF810 for signal conditioning. This will dictate the total number of DS250DF810 devices required for the board. It is generally recommended that channels with similar total insertion loss on the board be grouped together in the same DS250DF810 device. This will simplify the device settings, as similar loss channels generally utilize similar settings.
2. Determine the maximum current draw required for all DS250DF810 retimers. This may impact the selection of the regulator for the 2.5 V supply rail. To calculate the maximum current draw, multiply the maximum transient power supply current by the total number of DS250DF810 devices.
3. Determine the maximum operational power consumption for the purpose of thermal analysis. There are two ways to approach this calculation:
   1. Maximum mission-mode operational power consumption is when all channels are locked and retransmitting the data which is received. PRBS pattern checkers/generators are not used in this mode since normal traffic cannot be checked with a PRBS checker. For this calculation, multiply the worst-case power consumption in mission mode by the total number of DS250DF810 devices.
   2. Maximum debug-mode operational power consumption is when all channels are locked and retransmitting the data which is received. At the same time, some channels’ PRBS checkers or generators may be enabled. For this calculation, multiply the worst-case power consumption in debug mode by the total number of DS250DF810 devices.
4. Determine the SMBus address scheme needed to uniquely address each DS250DF810 device on the board, depending on the total number of devices identified in step 2. Each DS250DF810 can be strapped with one of 16 unique SMBus addresses. If there are more DS250DF810 devices on the board than the number of unique SMBus addresses which can be assigned, then use an I2C expander like the TCA/PCA family of I2C/SMBus switches and multiplexers to split up the SMBus into multiple busses.
5. Determine if the device will be configured from EEPROM (SMBus Master Mode) or from the system I2C bus (SMBus Slave Mode).
   1. If SMBus Master Mode will be used, provisions should be made for an EEPROM on the board with 8-bit SMBus address 0xA0.
   2. If SMBus Slave Mode will be used for all device configurations, an EEPROM is not needed.
6. Make provisions in the schematic and layout for standard decoupling capacitors between the device VDD supply and GND. Refer to the pin function description in Pin Configuration and Functions for more details.
7. Make provisions in the schematic and layout for a 25MHz (±100 ppm) single-ended CMOS clock. Each DS250DF810 retimer buffers the clock on the CAL_CLK_IN pin and presents the buffered clock on the CAL_CLK_OUT pin. This allows multiple (up to 20) retimers’ calibration clocks to be daisy chained to avoid the need for multiple oscillators on the board. If the oscillator used on the board has a 2.5 V CMOS output, then no AC coupling capacitor or resistor ladder is required at the input to CAL_CLK_IN. No AC coupling or resistor ladder is needed between one retimer’s CAL_CLK_OUT output and the next retimer’s CAL_CLK_IN input. The final retimer’s CAL_CLK_OUT output can be left floating.
8. Connect the INT_N open-drain output to an FPGA or CPU if interrupt monitoring is desired. Note that multiple retimers’ INT_N outputs can be connected together since this is an open-drain output. The common INT_N net should be pulled high.