At 100kHz, you should aim for a time constant below 500ns, that would give rise and fall times of about 1.5us

Hi Ian0,
The above sentence is not clear.

For example:
If 100Khz, T=1/f => 1/100k => 10us

But, the one which I calculated above and the one which you calculated in the previous post #20, is not same. How do you calculated 500ns and 1.5us?. Please could you explain to me clearly so that I can calculate accordingly.

Thank you Ian0.

 

At 100kHz, you should aim for a time constant below 500ns, that would give rise and fall times of about 1.5us

I think I understand half-way.

If the parameter are:
Low level Output Voltage = 0.2V @ 500uA => R=400 ohms
Input Capacitance = 5pF

Therefore,
The time constant = 5p*400 = 2ns

Is the time constant (2ns) is Right?. According to the above parameters.

 

I think I understand half-way.

If the parameter are:
Low level Output Voltage = 0.2V @ 500uA => R=400 ohms
Input Capacitance = 5pF

Therefore,
The time constant = 5p*400 = 2ns

Is the time constant (2ns) is Right?. According to the above parameters.

Yes, for a single device; but you must have at least 2 devices on the bus!

 

Yes, for a single device; but you must have at least 2 devices on the bus!

May I know why must have at least 2 devices on the bus?.

If the time constant is 2ns, then the SPI communication speed/frequency is T=1/f => 1/2n => 500Mhz, Am I correct?.

Which means that the SPI will communicate with the speed/frequency of 500Mhz. Am I correct?.

Please could you answer above three questions.

Thank you Ian0.

 

May I know why must have at least 2 devices on the bus?.

Because if you only have one, it will have nothing to talk to!

If the time constant is 2ns, then the SPI communication speed/frequency is T=1/f => 1/2n => 500Mhz, Am I correct?.

"Time constant" has a distinct meaning which is different from "cycle time" or "period".
"Time constant" refers to a time delay caused by a resistor and a capacitor, and comes from the equation for charging and discharging a capacitor
\(
V=V_0(1- e^\frac{t}{RC})
\)
The Time constant is RC. (You will see it referred to by the Greek letter τ) So, if a signal is changing from say 0V to 5V and it is being delayed by a RC circuit, after ONE time constant it will have completed 63% of the transition
\(
1-e^{-1} = 0.63
\)
After two time constants it will have complete 86% of the transition
\(
1-e^{-2} = 0.86
\)
and after three time constants it will have completed 95% of the transition
\(
1-e^{-3} = 0.95
\)
and then it is as good as completed.

So that there is plenty of time for the waveform to become stable, and clock edges to register, it should not spend too much time in transition and should spend enough time stable at 5V or 0V.

For that to be true, the period should be about 20 times as long as the time constant. t=20τ f=1/(20τ).

 

Hi Ian0,

From the above time contact (2ns), How to calculate that how many devices could be connected on a single SPI bus with the communication speed of 100khz?.