Summary of Sensor Interfacing using ATmega8 microcontroller
Connecting a 3.3V sensor to a 5V microcontroller risks damage without proper level shifting. This article explains using inline resistors on the MOSI and SCK lines to limit current, allowing internal clamping diodes in the 3.3V device to protect it. It notes that MISO connections require no components as the 3.3V output is safe for the 5V input, provided the receiving pin tolerates it.
Parts used in Sensor Interfacing Project:
- SCP1000-D01 pressure sensor
- PIC or AVR microcontroller (5V)
- ATmega8 microcontroller
- 10K resistors (R7 and R8)
If you’ve ever tried to hook up a 3.3V sensor to a 5V micro, you know what I’m talking about – connecting these two can be a problem! There are several ways in which a 3.3V device can be safely connected to a 5v microcontroller. This tutorial will detail a few ways in which this can be accomplished.
Below is an example of how NOT to connect a 3.3V device (like the SCP1000-D01 pressure sensor with SPI interface) to a 5V microcontroller (like a PIC or AVR running at 5V). Although you may experience normal operating conditions, the lifespan of your 3.3V device will be dramatically shortened.
Directly connecting the devices together (see above) will overstress the 3.3V device and eventually lead to device failure.
In general, there are many ways to interface these two SPI components together. There are many ICs available on the market that are specifically designed to translate between logic levels. We’ve never used any of them! Historically they’ve been very difficult to source and there are a few methods below that make interfacing logic levels fairly simple.
Inline Resistor –
This is the easiest and most rudimentary way to connect your devices together. Below is a generalized diagram using the inline resistor method.
R7 and R8 : 10K resistors are placed between the MOSI (Master Output Slave Input) line as well as the SCK (clock) line. These resistors will decrease the amount of current flowing into the 3.3V device. The internal clamping diodes within the 3.3V device will attempt to clamp the incoming signal to 3.3V thus protecting the rest of the 3.3V device. Clamping diodes are normally found on the input lines of low voltage devices.
A clamping diode is a diode used to limit the peak voltage on a line to a pre-determined maximum voltage. The resistor reduces the amount of current flowing through the diode by several orders of magnitude. Limiting the current will reduce the chances of the clamping diode being permanently damage. Without the resistors in place, much higher current will flow through the diode. These levels could destroy the clamping diode and in turn, damage the device.
Caution is needed with this approach as it is not guaranteed that your 3.3V device will contain clamping diodes. Although most devices have clamping diodes, if your device does not, then this inline resistor method will not work to reduce the voltage input.
MISO : You should notice that there is no resistor on the output of the 3.3V device (MISO). You shouldn’t need any circuitry between these lines to connect with your 5V device.
For more detail: Sensor Interfacing using ATmega8 microcontroller
- Why is directly connecting a 3.3V device to a 5V microcontroller problematic?
Direct connection overstresses the 3.3V device, shortening its lifespan and potentially leading to failure. - How do inline resistors protect a 3.3V device when connected to a 5V system?
Resistors reduce current flow into the device, allowing internal clamping diodes to limit voltage without being damaged. - Which signal lines require 10K resistors in this interfacing method?
Resistors are placed on the MOSI line and the SCK line. - Is circuitry needed on the MISO line when connecting a 3.3V device to a 5V micro?
No, there is no need for circuitry between these lines because the 3.3V output is safe for the 5V input. - What happens if a 3.3V device lacks internal clamping diodes?
The inline resistor method will not work to reduce the voltage input if the device does not contain clamping diodes. - Can logic level translator ICs be used instead of resistors?
Yes, many ICs are available to translate logic levels, though they can be difficult to source historically. - What specific sensor is used as an example in the article?
The SCP1000-D01 pressure sensor with an SPI interface is used as the example. - What type of microcontrollers are mentioned as running at 5V?
PIC and AVR microcontrollers are mentioned as examples of 5V devices.
