MCU pin driving 74HC fanout

[mayd85]

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]

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.

 

[mayd85]

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?

 

[FvM]

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.