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MCU pin driving 74HC fanout

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mayd85

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Hi,

I am using my MCU to drive select lines of ten 74HC series multiplexers connected in parallel. Each select line presents a maximum capacitance of 10pF as per the datasheet. Which means the MCU pin has to switch a maximum of 100pF. The select lines switch approximately every 250us. In this case, do I need to have a buffer to drive the select lines or just the MCU pins will be able to drive this load adequately?
 

FvM

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100 pF is still in a regular range and shouldn't cause problem. To reduce pin peak currents and power supply bounce, it would be a good idea to place a 50 to 100 ohm series resistor.
 
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    mayd85

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mayd85

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Thanks FvM.

For that matter, how much of capacitive load can any regular MCU pin drive? I mean, after how much load should I start thinking of buffers?
 

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A buffer can be necessary i you need to drive a capacitive load with fast edges, e.g. a PWM switching MOSFET, or a clock line.
 

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