Application Examples - Converting between 5V & 3V logic

This area of the Mirrorbow website provides applications information and answers some of the more frequently asked questions from customers 

Main Index - USB products resources - Ethernet products resources - Converting between 5V & 3V logic - Driving LEDs - Driving relays - Wireless Mains Control

Converting between 5V and 3V logic

The USB25IO product uses 5V logic while the EthernetIO product range uses 3.3V. So what do you do if you want to interface 3.3V circuits to USB25IO or 5V circuits to an EthernetIO product? Its pretty straight forward and this section tells you how.

Driving 5V inputs with 3.3V outputs: A 5V logic input device normally has a threshold for logic 1 of around 2.8V. This means that you can use the 3.3V outputs of the EthernetIO to directly drive a 5V logic input device (in fact this is exactly what happens on the Ethernet IO Display board, the display is 5V). Its important to keep connecting wires short and not to drive more than one input with a 3V3 output, otherwise the loading can cause the voltage at the devices input to drop and may cause problems.
On the Ethernet IO 5V tolerant ports (see below) as an alternative, if you find the 3V logic is not for some reason enough to drive the 5V circuit, you can connect a 10k resistor from the output to 5V, program the output to be a logic zero and then turn the output to an input for a logic 1 (will go to 5V through the resistor) and an output for 0 (will be driven low).

Driving 3.3V inputs with 5V signals: Normally you cannot directly connect a 5V logic signal to a 3V logic input! However, on the ethernetIO products Ports 1,2,3 and 6 are what is know as 5V tolerant. This means they can be driven from 5V logic signals without problems. Do not try to drive ports 4 and 5 directly with 5V signals! (Note: Port1 and 2 are used by the LCD display on Ethernet IO Display models). You can drive any 3.3V logic input from a 5V signal if you put a 1k2 resistor in series with the input and a 2k2 resistor to ground at the 3.3V logic input, this will convert 5V to approx 3.2V).