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Altera_Forum
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15 years ago

A problem on I2C, which may cause by the pin to pin resistance on the emulation board

Dear Sir,

I met one problem as the attachment. The voltage drop is supposed

caused by the pin to pin impedance, I guess. Because pin to pin resistance is about ony 30K to 40K Ohms which is much lower than that

in MAXII micro. The emulation board that I'm using is the DE2-70. When

I use the board to design an I2C to drive an external LCM through GPIO 0 and a ribbon bus. I've gotten a waveform as the attachment. After checking, I found the pin to pin resistance isn't as high as that in MAXII micro. I don't know how many ohms are appropriate in general? The pull-up resistors that I use are FPGA internal pull-up resistors. Suppose, the values of the internal pull-up are close to the pin to pin resistance.

Are there anyone has idea about the issue? Thank you!

Regards,

Peter Chang

2 Replies

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    15 years ago

    Hi Johannes,

    You're right. The high pull-up is one of the problem. But I'm using a FPGA

    internal pull-up. I'm not quite sure how large it is. I may only estimate it

    based on the voltage level I could see. I don't know if it is able to be set. Anyway, from the point of view, to solve the problem, I may only use

    an external pull-up. Do you think so? Thank you!

    Peter Chang
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    15 years ago

    From your draqing: Problably the pull up on the SDA/SCL line is too high. For most I2C busses the internal pull ups of fpga's are to high in value. A 4.7kOhm pull up on the lines is a good starting point. Or some device is loading the bus ...