Hi,
I have been trying to ask this question at NXP I2C support & Forum, but haven't had a reply so far. It is really on the I2C bus, rather than on any particular controller.
So, here we go:
I have a processor that supports the I2C port (Philips 2.1 Rev), and I am trying to measure the clock frequency for fast mode. I have an Rp (pull up R) of 1.2KOhm to 3.3V.
I read that I need to have a correct pull-up resistance, and it also depends on the capacitance. I have NO slave connected to the device, and I am probing the signals from the pcb using an oscilloscope, so can I assume a Max Cp of 400pF, or do I need to actually measure the rise-time and then look at it?
The formulae as I know is Rmax*Cmax = 1.18tr, where tr is the rise time (300ns for fast mode), Rmax is the limit outside which the clock starts slowing down, and Cmax is the max load capacitance.
I just want to arrive at accurate frequencies, with no slave device connected. Currently I am observing 345Khz for 400Khz. Could it be that the capacitance is too high (thus Rp should be rather low, so that the rise time doesn't increase), but I have nothing connected to the Master I2C signals?
The processor spec says it confirms I2C spec, and I dont have any other details from its datasheet. How exactly do I arrive at an accurate clock frequency for my Master board, when no slave is connected to it?
Thanks
Prasanth.
I have been trying to ask this question at NXP I2C support & Forum, but haven't had a reply so far. It is really on the I2C bus, rather than on any particular controller.
So, here we go:
I have a processor that supports the I2C port (Philips 2.1 Rev), and I am trying to measure the clock frequency for fast mode. I have an Rp (pull up R) of 1.2KOhm to 3.3V.
I read that I need to have a correct pull-up resistance, and it also depends on the capacitance. I have NO slave connected to the device, and I am probing the signals from the pcb using an oscilloscope, so can I assume a Max Cp of 400pF, or do I need to actually measure the rise-time and then look at it?
The formulae as I know is Rmax*Cmax = 1.18tr, where tr is the rise time (300ns for fast mode), Rmax is the limit outside which the clock starts slowing down, and Cmax is the max load capacitance.
I just want to arrive at accurate frequencies, with no slave device connected. Currently I am observing 345Khz for 400Khz. Could it be that the capacitance is too high (thus Rp should be rather low, so that the rise time doesn't increase), but I have nothing connected to the Master I2C signals?
The processor spec says it confirms I2C spec, and I dont have any other details from its datasheet. How exactly do I arrive at an accurate clock frequency for my Master board, when no slave is connected to it?
Thanks
Prasanth.