A microcontroller’s reset pin is held low by the voltage across a capacitor that…

PHYSICS · Multi-part · A Level · Physics · Fields and their consequences · Fields and their consequences

Problem

A microcontroller’s reset pin is held low by the voltage across a capacitor that charges through a pull-up resistor from a

3.3\,\text{V}

supply. The pin must remain below

1.2\,\text{V}

for at least

150\,\text{ms}

after power is applied to ensure reliable start-up. Available components on the production line are: • Resistors:

100\,\text{k}\Omega

150\,\text{k}\Omega

220\,\text{k}\Omega

• Capacitors:

1.0\,\mu\text{F}

2.2\,\mu\text{F}

4.7\,\mu\text{F}

(a) For each available resistor, calculate the time constant when it is paired with the

1.0\,\mu\text{F}

capacitor. (b) Using your results from part (a), identify which resistor–capacitor pair(s) meet the requirement that the reset pin stays below

1.2\,\text{V}

for at least

150\,\text{ms}

. Support your answer with calculations or clear reasoning. (c) The engineering team wants the smallest-valued capacitor that still satisfies the timing specification. State the specific resistor–capacitor combination you would recommend and justify your choice quantitatively. (d) Describe one trade-off, other than timing, that might arise if the design team decided instead to use the

4.7\,\mu\text{F}

capacitor with the

100\,\text{k}\Omega

resistor to obtain a similar delay.

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