Teeny PWM and Current Limiting Resistors

rmills

Active member
Shift Register Datasheet: https://www.ti.com/lit/ds/symlink/tpic6a595.pdf

I have this shift register tied to a Teensy 4.1, i'm driving the G (pin 4 on the TPIC) with a PWM pin (4 on the teensy) so it can act like a global fan speed control. Does the teensy have current limiting resistors on the pins internally or would I need to treat that like an NPN and add say a 1k series resistor to limit current?
 
I just realized that /G is inverted, is there anyway or a specific pin that can do an inverted PWM in software?
 
Quick notes ...

Pins don't current limit - needs to be done externally

To invert the output - just invert the desired on time? For 0-255 8 bit value instead of 25 for 10% on it would be 255-25?
 
The data sheet has the information to answer your question:

IIH High-level input current VI = VCC 1 μA
IIL Low-level input current VI = 0 −1 μA

1 micro ampere is a very small current, you will not need current limiting resistors.

On the other hand, this is a 5 volt device:

Logic supply voltage, VCC 4.5 5.5 V
High-level input voltage, VIH 0.85 VCC VCC V
Low-level input voltage, VIL 0 0.15 VCC V

0.85 * 5.0 volts gives a VIH of 4.25. The Teensy will drive to 3.3v high. It may work if driven by 3.3 volt logic, but it may not. You may need logic level converters.
 
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