Is 3.3V UART possible with IR Toy?

October 26, 2010, 04:28:22 pm

Hi! I need a 3.3V serial port, and IR Toy has one. I tried to look from PIC manual and everywhere, but found nothing interesting. Then I measured 5.0V between TX and GND. Is there a software way to use 3.3V logic level or should I try something else?

Re: Is 3.3V UART possible with IR Toy?

Reply #1 – October 26, 2010, 05:37:12 pm

The USB IR Toy is completely designed around 5V. 5V from the USB cable with no regulation or other source of power. All I/O pins on the PIC are going to be 5V, including the UART.

About the only thing you could try without rewiring the board is to rig a USB cable with 3.3V instead of 5V, and see whether that works. I kinda doubt it.

Re: Is 3.3V UART possible with IR Toy?

Reply #2 – October 26, 2010, 06:42:41 pm

I think you should be able to read the 3.3V RX line at with the USB Toy at 5V. It's high at 3.3V should give you enough margin to be read correctly. The Transmit line from the Toy is what you'd need to drop to 3.3V. If you were determined to do it with the Toy you could drop the 5V to 3.3V with a resistor (to limit current) and a 3.3 or 3.6 V Zener diode to shunt the voltage above the 3.3V to ground. Alternatively I think you could probably do the same thing on the USB 5V line as rsdio suggested. I'm an electronics novice so others will hopefully correct me.

Alternatively FTDI cables/breakouts from
* Seeed Studio UartSB -s switchable 3.3 or 5 V
* Adafruit FTDI Friend also switchable)
* Sparkfun FTDI order either 3.3V or 5V.
* FTDI TTL-232R-3V3, 6 pin 3.3V cable, used for some Arduinos and others.

The Seeed Studio UartSB is probably the most useful since it can double as an XBee adapter. Also the latest version has a 2 x 3 socket to make it easy to do FTDI AVR bit bang programming.

Unfortunately many of the FTDI breakout boards only give you 6 pins and you get either RTS or DTR but not both. For bit banging you'd like access to as many of the pins as you can. I believe there will be a Dangerous Prototypes FTDI bit bang board for high(er) speed programming/jtag that might also be useful for those 3.3 or 5V embedded uart applications.

I don't remember if the Bus Pirate can do 3.3V uart, if it can that might be your best bet.

EDIT: The Bus Pirate does do 3.3V for async uart operations, http://dangerousprototypes.com/docs/UART