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In my I2C class I have an _init() method that, in part, looks like this: void SMD_I2C_Device::_init(uint8_t address, unsigned long busSpeed) Which is 72, which we’ll need as an unsigned integer.
![wire library set i2c clock speed wire library set i2c clock speed](https://electropeak.com/learn/wp-content/uploads/2021/04/GY-87-Pin-2-768x576.jpg)
If we assume, say, a microcontroller clock speed of 16MHz and a prescaler value of 1, this becomes: In your code, you should have F_CPU defined somewhere anyway, so for an SCL frequency of 100kHz, the formula would be: If you prefer to rely on operator precedence, be my guest.) (You’ll notice that I like to use lots of parentheses. Let’s rearrange this by multiplying both sides by (16 + (2 * TWBR * Prescaler) ) and dividing by SCLfreq:ġ6 + (2 * TWBR * Prescaler) = F_CPU / SCLfreqĢ * TWBR * Prescaler = (F_CPU / SCLfreq) – 16Īnd now, to get the TWBR that we’re after, divide both sides by (2 * Prescaler):
![wire library set i2c clock speed wire library set i2c clock speed](https://i.stack.imgur.com/AwKTe.png)
F_CPU is also a known – it’s the clock speed for your microcontroller. The SCLfreq is something we already know – it’s the SCL clock frequency we want to use. That still looks a bit ferocious, so let’s break it down and also rearrange it, because the thing we’re after (TWBR) is buried. SCLfreq = F_CPU / (16 + (2 * TWBR * Prescaler) ) It’s set using the TWBR register and you calculate it using the formula: You need to set the bit rate, which is the frequency of the SCL clock. Achieving the bit rates necessary for the very fast I2C modes is beyond the capabilities of the ATMEGAs and ATTINYs I like to use. I’m going to assume we’re dealing with devices that operate at either 100kHz or 400kHz. The master device on an I2C bus is responsible for creating the clock pulses on the SCL line.
#WIRE LIBRARY SET I2C CLOCK SPEED SERIAL#
Some devices, like the ATTINY84A that I’m currently messing about with, have an I2C mode, but via a Universal Serial Interface (USI) that is controlled in a different way and none of our concern – for now, anyway. So let’s deal with that now.īy the way, I’m dealing here with microcontrollers such as the ATMEGA328P that have proper I2C interfaces and the registers to go with them. Sticking it in a class method or function makes a lot of sense. I mention this because there’s one bit of calculating you need to do with I2C, to set the bit rate for the clock, that is easy to mess up. Yes, you can do this with functions too, but I like the ‘black box’ style of encapsulation of classes. One of the reasons I prefer using C++ to C is that you can write classes that hide much of the complexity. But once you get your head around a few concepts then it becomes much easier. There is a bit more complexity to using I2C compared to SPI.
#WIRE LIBRARY SET I2C CLOCK SPEED SERIES#
With this series of posts, we’ll be looking at using an AVR ATMEGA or similar in master mode. As we’ll see, the AVR’s registers and bits used to manage this bus mostly start with the letters ‘TW’. In fact, I2C requires just two lines – a two-way data line (SDA) and a clock line (SCL) – which is why it is also known as the Two-Wire Interface (TWI), especially by manufacturers who don’t want to pay NXP’s licence fee. And it’s also possible to have slaves interact with multiple masters, although that’s beyond the scope of this current discussion. Unlike SPI, where you need a separate line (and GPIO pin) for each device in order to activate it, with I2C you just preface traffic with the the device’s address (a one-byte number) to select which device should respond.
![wire library set i2c clock speed wire library set i2c clock speed](http://imgs.inkfrog.com/pix/ebayimage2012/24767-4.jpg)
The first is that the slave devices on the bus are addressable. Faster versions are available – the 400kHz fastmode and even 1MHz, 3.4MHz and 5MHz options – but the original slow speed is still the most commonly used by tinkerers and hobbyists like me.īut it does have some advantages. The bus isn’t staggeringly fast, at a base standard of 100kHz. The name is pronounced I-squared-C and is more properly written I☬, but I2C is easier to type. And second, there are lots of fascinating devices out there, including displays and sensors, designed to work with it.Ĭreated by Philips Semiconductor, now NXP, the Inter-Integrated Circuit bus was designed to connect devices, and particularly chips, in close proximity. When I first started playing around with Arduinos I quickly grew to like the I2C bus, and for a couple of good reasons.
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