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Original question:

CDCDB803: selectable SMBus addresses consult

CDCDB800: SMBUS and OE mode control

Chi Zhang2
Intellectual 2690 points
Part Number: CDCDB800

Hi Expert, 

my customer want to use CDCDB800 in server. but we have some questions for SMBUS and OE control. 

DB800 could support SMBUS and OE pin control clk[7:0], pin 46 could control SMBUS mode enable or disable, default is pull-down to enable SMBUS mode. Customer want to use SMBUS and OE control function in different scenario. so we won't connect pin46 to VDD and disable SMBUS mode. 

my question is

1. what is the priority for OE control and SMBUS control?

2. if we make pin46 floating(SMBUS enable status), could OE# control be active? or must disable pin46. 

3. if we don't use OE# control, want to use SMBUS control instead, how to connect OE#, disable or enable? 

actually, customer want to use SMBUS and OE pin control function, so need a compatible BOM connection. thanks. 

BR

Chi

  • 0
    TI__Genius 9260 points

    Hi Chi,

    CDCDB800 supports both SMBUS and OE pin control. You can use each of them individually or combined. Section 9.2.2.1 of datasheet explain how they function in each mode.

    Table 8.1 explains different all pins and register combinations for clock outputs.  

    Chi Zhang2 said:
    what is the priority for OE control and SMBUS control?

    SMBUS and OE have a AND function between two control types so both are needed in certain conditions for other to operate as mentioned in the table. 

    Chi Zhang2 said:
    if we make pin46 floating(SMBUS enable status), could OE# control be active? or must disable pin46. 

    Pin 46 is for SMBUS write lock. So if you have that pin floating then OE# control would still work because default values for registers are set to 1.

    Chi Zhang2 said:
    if we don't use OE# control, want to use SMBUS control instead, how to connect OE#, disable or enable? 

    You can leave them floating or tie through a pull down to ground. External pull down option is good when there is possibility of noise getting coupled to OE# pins.

    Chi Zhang2 said:
    actually, customer want to use SMBUS and OE pin control function, so need a compatible BOM connection. thanks. 

    You can connect output through a jumper to VDD or GND or a GPIO pin for controlling pins. Pin 46 can be left  floating but it you are concerned that noise can get coupled on this pin and cause a WRITELOCK then its better to tie it to GND externally.

    Let me know if this answer your questions.

    Best,

    Asim

  • 0 in reply to Asim Khan
    TI__Intellectual 2690 points

    Hi Asim, 

    thanks for your reply, it is very clear. 

    OE and SMBUS have a AND logic inside, do we also have the protect inside to prevent the situation that some noise coupled on the OE pin? customer mentioned that Renases has this protection inside. 

    how about the input current spec for OE pin, GPIO only could drive 3-5mA max, could we connect GPIO directly to OE pin? thanks. 

    BR

    Chi

  • +1 in reply to Chi Zhang2
    TI__Genius 9260 points

    Hi Chi,

    I am not sure what protection you are referring here. We have a internal pull down on these pins so if noise doesn’t pass the threshold voltage if should be okay.

    GPIO should be able to drive these pins with that current specifications.

    Best,

    Asim