topic Re: Sinking 5V external LEDs through the 3.3 V MCU on the Freedom board? in Kinetis Microcontrollers

Sinking 5V external LEDs through the 3.3 V MCU on the Freedom board?

injunear1 — Mon, 12 Nov 2012 13:51:55 GMT

I have four external LEDs connected to pin 10 of J9 (USB +5V). It is my intention to turn them on by providing ground through pins 9, 11, 13, & 15 of J9 (PTE2, PTE3, PTE4 & PTE5 respectively). I am able to turn them on as desired and the logic to turn them off is also working. However, when off they are still dimly lit. Could this be because when high (LED OFF mode) the CPU pins are at 3.3 volts and they are still sinking the LEDs attached to 5V from the USB?

Could this be damaging the CPU?

From what I have read so far, the CPU is engineered very well so I hope this kind of potential electrical error has not baked the silicon.

Nevertheless, is this why my LEDs are still slightly on when I want them to be off?

What would be a good way to work around this issue? Should I source all 4 of them at pin 4 or 8 P3V3? What is the current capability of P3V3?

Thank you.

BTW, Freescale - I think you have hit a home run with this eval board. That includes the price. I can imagine countless adolescents and wannabees (as well as innumerable professionals) hacking this board to pieces because of its versatility and cost.

(Perhaps you can offer a kit for under $30 that includes headers on the board and an IO box with a few LEDs and switches, and a getting started manual to explain the details of making it all work together.)

Re: Sinking 5V external LEDs through the 3.3 V MCU on the Freedom board?

injunear1 — Mon, 12 Nov 2012 14:15:59 GMT

I went ahead and switched my LED sourcing from pin 10 to pin 8, the LEDs worked as expected. I guess that was the problem.

But the question remains: How much can P3V3 source? Where would one find that in the documentation?

Thank you.

Re: Sinking 5V external LEDs through the 3.3 V MCU on the Freedom board?

JimDon — Mon, 12 Nov 2012 15:21:20 GMT

You should look thru this document to answer your questions. In freescale, the basic electrical specs are in the data sheet:

http://cache.freescale.com/files/32bit/doc/data_sheet/KL25P80M48SF0.pdf?fpsp=1

Re: Sinking 5V external LEDs through the 3.3 V MCU on the Freedom board?

injunear1 — Mon, 12 Nov 2012 20:26:49 GMT

Thank you for the direction to look. If I am understanding the data sheet correctly, the answer is 100mA total for all output pins and 125mA for all pins and related paraphernalia.

Thank you.

Re: Sinking 5V external LEDs through the 3.3 V MCU on the Freedom board?

peterdvorak — Wed, 05 Dec 2012 20:11:57 GMT

RE:How much can P3V3 source?

The USB limit is up to 500mA

The onboard regulator limit is about 500mA depending on cooling.

The biggest limits are uC I/O pins and total package limits as published in spec sheet.

Peter