[PIC] I2C Bus Speed Requirements

[VirusX2]

Hello everyone, i'm old reader but new member in this forum..!!

I'm currently working on a Real-time measurement system to measure voltage, current, frequency and some other electrical signals. The heart of my system is a PIC18F4553 and as communication protocol i must use I2C cause for every measurement there will be a subsystem on separate PCB. My problem is what bus speed i really need..? 400 KHz or i must go to 3.4 MHz. I want basically to communicate with A/D converters. The fastest i found produces 189 kSPS (Samples/Second) so can take sample every 5.2 μS. It's supports all I2C speeds. How can i calculate what bus speed i really need?

 

[udayan92]

The method explained here is for finding data bus speed(s) in CPUs.The connection(s) between the CPU peripherals connected by the bus are not very different from the connection(s) you are having in your measurement system.It should give you a basic idea.
https://www.smxrtos.com/articles/whiteppr/flashperformance.htm

 

[FvM]

There aren't much I2C devices that support 3.4 MHz "Hs" mode. PIC processors e.g. don't.

 

[shaiko]

How can i calculate what bus speed i really need?

Your PIC has an SPI peripheral that can go faster then I2C - why not use it instead?

 

[VirusX2]

There aren't much I2C devices that support 3.4 MHz "Hs" mode. PIC processors e.g. don't.

Are you sure..? I am programming my PIC with 3.4 MHz speed at I2C parameters and no error occurs.

Your PIC has an SPI peripheral that can go faster then I2C - why not use it instead?

Cause i need the hot-plug and the build-in acknowledgment functions of I2C bus..

 

[FvM]

I am programming my PIC with 3.4 MHz speed at I2C parameters and no error occurs.

Maybe it works by chance, but not according to PIC specification. Hs-mode uses special push-pull drivers which are not provided by PIC hardware.

 

[VirusX2]

Your PIC has an SPI peripheral that can go faster then I2C - why not use it instead?

What type of cable is suitable for SPI communication (at 2.5 - 5 MHz), max length about 30-40 cm? I am thinking about UTP CAT5. I want this to connect the main pcb with the sub-pcb where the ADC is located.

 

[Easyrider83]

I really wanna take a look on PCB that have I2c working on 3MHz. Too much bus capacity.

 

[VirusX2]

I really wanna take a look on PCB that have I2c working on 3MHz. Too much bus capacity.

I know that.. Im asking about SPI as you see in my last post..!!

 

[Easyrider83]

For 5Mhz it is no difference. It will work fine.
Try to trace signal wires same lenght as possible.

 

[VirusX2]

For 5Mhz it is no difference. It will work fine.
Try to trace signal wires same lenght as possible.

Is it possible to have instability cause of twisted pairs of the UTP cable..?

 

[Easyrider83]

Sure. Twisted pairs designed for differencial pairs. In your case beter use flex cable.