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TPS65987D: SPI Flash programming - signals driving against each other

Florian Kaffanke
Prodigy 170 points
Part Number: TPS65987D
Other Parts Discussed in Thread: TMUX1511

Tool/software:

Hello,

I work on a design that uses TPS65987D for USB PD. The schematic is very similar to evaluation module TPS65987EVM. Like in the EVM we want to program the SPI Flash attached to the TPS65987D directly. In the EVM, both the TPS65987D and FTDI USB controller (a microcontroller in my case) are connected to the same SPI Flash signals. This leads to two questions:

  1. There are two SPI masters on the same bus, don't they interfere with each other? I guess not, because the SPI interface from the TPS65987D is high impedance after booting up and reading the flash only once. But this only my guess. I couldn't find any reliable information about this.
  2. On my design the TPS65987D is the first part that powers up. The yet unpowered microcontroller on the SPI bus might interfere with the SPI signals. To prevent this I added a TMUX1511 to the microcontroller side of the SPI bus. Is this a reasonable solution?

Best regards,

Florian

  • +1
    TI__Mastermind 37855 points

    Hi Florian,

    Florian Kaffanke said:
    There are two SPI masters on the same bus, don't they interfere with each other? I guess not, because the SPI interface from the TPS65987D is high impedance after booting up and reading the flash only once. But this only my guess. I couldn't find any reliable information about this.

    I think they do not interfere due to controlled usage of the SPI masters. On the EVM, the 987 is only master on initial boot when reading, and then does not use the SPI lines. The FTDI only uses the SPI lines when the GUI initiates a SPI Erase/Read/Write. This typically happens much later than boot. The SPI lines on the 987 should be high impedance after boot and reading, and it is safe for another device to act as SPI master on those lines after the boot and initial read as seen on the EVM.

    Florian Kaffanke said:
    On my design the TPS65987D is the first part that powers up. The yet unpowered microcontroller on the SPI bus might interfere with the SPI signals. To prevent this I added a TMUX1511 to the microcontroller side of the SPI bus. Is this a reasonable solution?

    Yes that should be fine, as long as the TMUX does not interfere. One comment I have is that the FTDI is primarily for the EVM, and usage with the GUI. On a customer's design, we don't typically expect them to use the FTDI unless they are using it for things other than the PD controller. 

    Thanks and Regards,

    Chris

  • 0 in reply to Christopher Lim1
    Prodigy 170 points

    Hi Christopher,

    thanks for your answer. Regarding the FTDI chip: we're not using one. But we already have a microcontroller in our design. So the easiest way for writing to the flash is by using the existing microcontroller. This saves us another programming header.

    Best regards,

    Florian