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ADuC706x: weak internal pull-up strong enough for ADuM241E's input?

Question asked by m.teichgraeber on Apr 19, 2016
Latest reply on Apr 21, 2016 by MMA

When measuring the voltage on pins of an ADuC7061 that are configured as digital inputs with internal pull-up enabled (the default for many pins after Reset), I'm observing values between 1.8 to 2V, depending on the DMM/oscilloscope used, while DVDD is at 2.5V. Given the input resistance of the voltmeters, this suggests a value of a few MΩ of the internal pull-up resistor.

I suppose this is what I should expect; the answer to a question regarding the ADuC7023 most likely applies to the ADuC706x as well:


I'm using the ADuC7061 as an SPI-Slave connected to an SPI master via an ADuM241E isolator. What I want to make sure is, that after an ADuC7061's software reset (fixing er007) the MISO pin will be at 'H' level even for the short duration when it still is a digital input, so that the SPI master -- unaware of the ADuC7061's reset -- can recognize this condition by reading all bits as '1'.

Will the internal, weak pull-up resistor of an ADuC7061's MISO pin (when still configured as a digital input) be enough to guarantee a logic high at the ADuM241's input? This device has a logic high threshold VIH of 0.7xVDDx, in this case 1.75V. The data sheet says, the isolator's maximum input current II per channel may be 10µA. On an ADuC7061's input pin I have observed a current of ≈10µA at a voltage of around 1.6V – which would be below the ADuM241's VIH.

Is it correct that under this conditions an external, stronger pull-up resistor on the MISO line is required?