TWI clock pull up

RE: TWI clock pull up

Paolo A. — Wed, 30 Aug 2017 15:40:48 GMT

Hi everyone, about this question I have some perplexity too.

If I set the pin as S0D1(like Ole suggest) the clock signal has a slow slope (as if it were filtered). If I set the pin as S0S1 the clock signal has a steep slope (like a step signal). I use the TWI0 with 10KOhm pull-up resistor, without internal pull-up, and pin drive as S0S1. So, the answer's Ole and datasheet say the truth, but for me the behaviour is better if I set the pin like S0S1 instead of S0D1 (like Aras said).

What is the right way to take? Thanks.

RE: TWI clock pull up

Ole Bauck — Tue, 15 Nov 2016 12:17:48 GMT

Sorry for the late answer, the slave should pull both lines.

RE: TWI clock pull up

aras — Sat, 05 Nov 2016 11:39:29 GMT

The slave pulls the DATA, not clock. Is this correct?

RE: TWI clock pull up

Ole Bauck — Mon, 12 Sep 2016 14:55:06 GMT

Now I understand. You cannot set the drive to S0S1. If you do you will have a short when the slave pulls the clock down to ACK the master. I will update my original answer with more info.

RE: TWI clock pull up

aras — Thu, 08 Sep 2016 14:05:10 GMT

I verified with the slave device removed. No pull-up. No other connection. Exactly the same result. Chip is N51822, CFACA0, 1506AK

RE: TWI clock pull up

Ole Bauck — Tue, 06 Sep 2016 13:58:53 GMT

You are sure the slave does not have internal pull-ups? Try measuring without the slave attached.

RE: TWI clock pull up

aras — Fri, 02 Sep 2016 16:10:35 GMT

I'm not so sure about that.

I set up my clock pin as follows:

  NRF_GPIO->PIN_CNF[twiInstance[twiHandle].clockPinNumber] =     \
    (GPIO_PIN_CNF_SENSE_Disabled << GPIO_PIN_CNF_SENSE_Pos) \
  | (GPIO_PIN_CNF_DRIVE_S0S1     << GPIO_PIN_CNF_DRIVE_Pos) \
  | (NRF_GPIO_PIN_NOPULL    	 << GPIO_PIN_CNF_PULL_Pos)  \
  | (GPIO_PIN_CNF_INPUT_Disconnect  << GPIO_PIN_CNF_INPUT_Pos) \
  | (GPIO_PIN_CNF_DIR_Output      << GPIO_PIN_CNF_DIR_Pos);

There is no external pullup. Attached is the scope trace of the clock pin.

IMG_20160902_102434.jpg

Of course, this only works if there is only one master and the slave device does not do any clock stretching.

RE: TWI clock pull up

Ole Bauck — Thu, 01 Sep 2016 13:01:35 GMT

You need to have some pull-up resistors on the lines. The lines on TWI (or i2c) are "open drain", meaning that they can only be driven low. If you omit the pull-up resistors, the line will float when it is not driven low.

Since the lines can only be driven low you should use S0D1.

The internal pull-up on nRF51 is about 13KOhm. This is a bit too high for TWI and will give problems when running at 400KHz (the clock will be skewed due to the RC circuit the lines create). Recommended pull-up is about 4.7KOhm.

EDIT 2016.09.12:

You should not use S0S1. Then you will have a short when the slave pulls the line low for ACK. The configuration should be as defined in the Reference Manual: