TM4C1294XL PWM

Mike Pascucci said:

I'm trying to find the spec on what voltage level the PWM inputs can accept.

Might many here seek similar?   Most of us share the belief that there are PWM Outputs - I cannot recall any PWM pins specifically designated as Inputs.

Now there ARE Timer pins - which may serve to effectively process PWM input signals.   The rear of the MCU manual should guide you as to signal level requirements for such "Timer" inputs.

Thank you for your reply. I'm afraid I am not very familiar with this, and might refer to things incorrectly. And I should have been more specific - I'm using several of the PM pins of the BoosterPack 2 headers as inputs for my PWM signals. In the User's Guide for the Launchpad, some of them have their Standard Function listed as PWM (perhaps typically used as an output?), and others are Capture or GPIO.

I'm guessing some of these might be the timer inputs you're referring to (?), but I don't see anywhere in the guide where signal level requirements are shown for any inputs - timer or otherwise. In the Specifications table, there's the Board Supply Voltage and Break-out Power Output, but nothing about input signal level.

Am I looking in the wrong place? My software guy is much more familiar with it than I am - he coded it up, so it is currently working, but he's not available to ask. I had hoped it'd be listed fairly obviously in the manual, but I must be missing something...

Mike Pascucci said:

I'm using the TM4C1294XL Tiva Launchpad, and I'm trying to find the spec on what voltage level the PWM inputs can accept.

The schematic for the EK-TM4C1294XL shows the pins of the TM4C1294NCPDT directly connected to the boosterpack and breadboard connectors without any level-shifting.

Mike Pascucci said:

Can it be 5V, or does it need to be stepped down to 3.3V?

Section 27.14.1 Types of I/O Pins and ESD Protection of the TM4C1294NCPDT datasheet says:

Caution – All device I/Os pins, except for PB1, are NOT 5V tolerant; voltages in excess of the limits shown in Table 27-6 on page 1820 can permanently damage the device. PB1 is used for the USB's USB0VBUS signal, which requires a 5-V input.

Table 27-1 shows that the absolute maximum input voltage is 4V.

Therefore, any external 5V signals will need to be level-shifted to 3.3V before connecting to TM4C1294NCPDT input pins.

Note that it IS the specific MCU data-sheet which proves, "Judge/Jury" as to, "Acceptable Input Signal Levels." Any (other) data would be of far less value than that prescribed by the MCU's sheet.

You need to insure that ALL input signals (not those just destined for PWM) ARE limited to 3V3.  (but for PB1, as another noted)    If not - and if your usage has been brief - I'd STOP usage immediately - and confine those signals to the MCU's 3V3 level.  

Simple voltage dividers or dedicated, voltage level shift ICs - well serve such purpose...

I will level shift these, and any other, input signals to 3.3V. Thank you both for your responses - very helpful and appreciated.