LPC55Sx Pullup, Pull down resistor range

[RBhagat]

Hello, 

With regards to Pull-up, Pull-down values, we are seeing some discrepancy between the eval board (PU's have 100K, PD's 2.2K) and the AN13033 (10K).

Based on datasheet we understand internal pull-up/ pull-down resistor value is in the range of 40K-62K (refer snapshot). We would like to know for all pins which are used in application if we can use PU/ PD resistance values from 2K-10K instead of 100K. Will there be any damage to LPC55S6x uC if stronger PU/ PD values are used.

Kindly do revert.

Thanks,

RB

[xiangjun_rong · Accepted solution]

Hi, Roma,

Regarding the pull-up or pull-down resistor, I think we have to discuss it based on two conditions, one is input pin another is output pin.

For input pin, the pin has high impedance, as the data sheet, the IIH and IIL specs are 200nA at most, so the minimum impedance is 3.3V/200nA=1.5*10**(7) ohm. Because the 3.3V or GND can be connected to the input pin directly, therefore there is not bottom value for the pull-up or pull-down resistor in the input pin case. For the top resistor value, because the input pin can be float, so there is not top value for the pull-up or pull-down resistor. But if the pull-up or pull-down resistor value is too large for example 100K ohm, if there is not the other external source to drive the input pin, maybe you can not get the correct logic. for example, if you connect a 100K ohm pull-up resistor to a GPIO input pin without the other driving source to drive the same input pin, when you read the logic, maybe you can not get the 1 logic. In the case, 10K ohm resistor is recommended so that you can get logic 1.

For the output pin, if you connect a large pull-up or down resistor for example 100K ohm, it is okay. But if you connect for example in the extreme 100 ohm pull-up resistor to the output pin, because the IOH and IOL are 4mA, the 3.3V/100ohm=33mA, of course, the 33mA current is enough to damage the pin. So for the output pin, you have to use large pull-up or down resistor so that it does not take effect on the output logic.

In conclusion, for the input pin without the other driving source, you have to select a small pull up or down resistor so that you can get a determinate logic. If the input pin has the other source, the pull up or down resistor must be large lest it takes effect on the driving source logic.

For the output pin, you have to use large pull up or down resistor value, even do not use it.

Hope it can help you

BR

XiangJun Rong