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Original question:

TCA9617A: IIC pullup resistor value

TCA9617A: TCA9617A 上升沿时间

Chambin Zhang
Prodigy 165 points
Part Number: TCA9617A
Other Parts Discussed in Thread: TCA9803, TCA9617B

你好，

我有一块电路板，通过TCA9617A 作为IIC通讯的芯片。

靠近MCU侧（主侧）的使用2.2K电阻上拉，靠近从设备侧使用4.7k电阻上拉，我测试波形，得到主侧波形的上升沿约为150ns, 从侧波形的上升沿时间约为1us.电路如下：

我想把从侧的上升沿时间调小，把电阻改小，改为1.2K, 但是上升沿时间基本没有变化，而且略有增加，变为1.1us，不知道是什么原因？

还有个问题是主从两侧的上升沿时间是否满足IIC协议要求？

如果我想把从侧上升沿时间调小，有什么方法？

谢谢。

  • 0
    TI__Genius 15345 points

    Hello,

    Can you translate this post into english?

    Best,

    Chris

  • 0 in reply to Chris Ayoub
    Prodigy 165 points

    Hello，

    We use TCA9617A as IIC IC and set master side pullup resistor as 2.2K (rise time is about 150ns)and slave side pullup resistor as 4.7k(rise time about 1us), please see my design schematic:

    .

    I change slave pullup resistor value as 1.2K to reduce rise time, but actually the rise time is about 1.1us, almost no change.

    I don't know why change pullup resistor can't change rise time?

    Is correct value of rise time our schematic setting?

    How to make slave side rise time smaller?

    Thanks.

  • 0 in reply to Chambin Zhang
    Guru 317370 points

    Are you measuring the rise time of the signal itself, or the propagation delay?

    To reduce the rise time, decrease the pull-up resistor value further, or decrease the capacitance of the bus, or add an I²C accelerator. If you are using no more than 400 kHz at no more than 3.3 V, you could use the TCA9803 (a buffer with integrated rise-time accelerator).

  • 0 in reply to Clemens Ladisch
    TI__Genius 15345 points

    Hi Cambin,

    Can you provide a scope shot of your rise time before and after the pull up resistor change?

    Best,

    Chris

  • 0 in reply to Chris Ayoub
    Prodigy 165 points

    Hi Chris,

    Please see waveform as follow:

    Before change

    Slave side 4.7k pullup resistor

      

    Master side 2.2K resistor

    After change 

    Slave side 3.9K

    Master side 2.2k keep no change

    Thanks.

  • 0 in reply to Chambin Zhang
    TI__Guru 74292 points

    Chambin,

    Chris is out of office the next two days but well make sure to address this thread while he's out.

    Regards,

    Eric Hackett 

  • 0 in reply to Eric Hackett
    TI__Genius 15345 points

    Cambin,

    Can you actually zoom in on a rising edge? I just want to make sure that the rise time was calculated correctly.

    What devices/device is connected to the B side of the device? It also looks like communication is fine. What speed are you trying to communicate at? What is a rough estimate on your bus capacitance on the B side?

    From your scope a 4.7k pull up to a 3.9k pull up is not that significant of a change. What happens if you use a 1.5K pull up?

    I also recommend using the TCA9617B. This version of the device comes with power off high impedance and you won’t risk accident back biasing your voltage rails.

    Best,

    Chris