Electronic – Disadvantage of using low resistor value for pull up (Strong pull up)

i2cpicpullupresistorssmbus

I was trying to choose a resistor value for a pull-up resistor I will use in my project. The project will have I2C communication and SMBus communication. When I was searching on the internet, I have seen some sentence that says using low resistor value for a pull-up resistor causes higher energy consumption. Is it the only disadvantage? If the only advantage of using a pull-up resistor is to reduce energy consumption, would the SMBus or I2C communication work without any pull up resistor? And how much energy does it save if I use 2 k pull up resistor, would it reduce consumption drastically? Thanks beforehand.

Best Answer

You do need a pull-up. And 2k is a low value so it will consume more power than 10k would. You need to select your pull-up to have the rise time small enough to be within specifications that depend on the clock speed, bus capacitance, current driving ability of the chips and their voltage levels to determine bus being high or low.

Related Solutions

Electronic – How is the clock frequency established between master and slave in I2C protocol

Following up the comments:

Yes, the frequency is hard-coded into some specific I2C-related registers. At runtime.

That said, your Arduino library might be doing some probing and rise time measurement on the bus for determining a viable clock rate. Let's see.

After doing a bit of source-digging, the answer is in twi.h

#ifndef TWI_FREQ
#define TWI_FREQ 100000L
#endif

Another piece from that very file:

TWBR = ((CPU_FREQ / TWI_FREQ) - 16) / 2;

Where TWBR stands for Two Wire Baudrate Register I suspect.

I'd call that enough evidence and definitely say, yes your I2C frequency is set directly by your library without any negotiations. If you want to change it, I'd suggest you #define TWI_FREQ before you #include twi.h (or indirectly through Wire.h)

Electronic – I2C communication won’t start unless SDA pullup resistor is reinserted

Your problem is that you have sprinkled i2c_wait() commands through your code.

This is a microcontroller, it is simple, it does exactly what you want. The state doesn't need to be constantly checked. You should only need to call i2c_wait() after interacting with SSPBUF. Have a look at http://www.8051projects.net/wiki/I2C_Implementation_on_PIC

By holding SCL high and toggling SDA (removing and replacing the resistor) you are sending a start bit followed by a stop bit. It isn't clear why this fixes the registers you are checking, as Chris Stratton suggested you could dump out the registers if you really cared.

Best Answer

Related Solutions

Electronic – How is the clock frequency established between master and slave in I2C protocol

Electronic – I2C communication won’t start unless SDA pullup resistor is reinserted

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