Crystal oscillator confusion, i2C communication & theory of operation

Hello,

I was wondering, suppose I would create a circuit that consist of three i2C sensors (temperature sensor, energy meter & Real Time Clock) interfaced with an MCU. The datasheet of the energy meter (ADE7953) & RTC has different specified crystal values.

It is my first time to do i2c communication involving several sensors and I am confused on the following things:

1. Since the sensors have different specified crystal oscillator value, does this mean I have to provide each of them a separate crystal? Is it important for it to do its function?

2. Is it possible to just provide one crystal oscillator for the entire circuit as reference since all the components will be communicating in i2C (interconnected with two wires only) and controlled my the MCU?

I know that there is a standard 100kHz, fast 400kHz i2c communiction, etc. so why do these components have specified crystal? Please enlighten me.

hi ee,
The ADE7953 datasheet states a xtal of 3.58MHz is required.
Which type are the Temperature and RTC modules.??
Most RTC modules require a particular frequency xtal.

The timing of the I2C communications is not directly tied to the xtal frequency of the peripheral modules.

E

esp1 said:

hi ee,
The ADE7953 datasheet states a xtal of 3.58MHz is required.
Which type are the Temperature and RTC modules.??
Most RTC modules require a particular frequency xtal.

The timing of the I2C communications is not directly tied to the xtal frequency of the peripheral modules.

E

Click to expand...

Thank you sir for your response.

It is the PCF8563 and as written on its spec sheet it is based on a 32.768 kHz frequency.

As i understood it, your last sentence means that communication frequency (SCL) of i2C is independent with the required clock frequency of the sensors for it to do its function?

I reviewed the Temp sensor and it doesn't need any clock input. Thank you again.

And Oh by the way, the MCU i picked is compatible with 3.58Mhz, so is it possible to supply both the ADE7953 and MCU with a single 3.58Mhz crystal? I was thinking of connecting it in parallel.

And Oh by the way, the MCU i picked is compatible with 3.58Mhz, so is it possible to supply both the ADE7953 and MCU with a single 3.58Mhz crystal? I was thinking of connecting it in parallel.

Click to expand...

May work, but not necessarily. Oscillator circuits and their bias voltages may be incompatible. You can make your live easier by providing separate crystals.

Re: Crystal oscillator confusion, i2C communication & theory of operation

Hi,

* RTC crystal has nothing to do with I2C. It is used for pre ise clock operation. Often a 32768 Hz crystal is used. Crystal oscillator needs to operate even on power diwn ...from a battery.

* energy meter crysral has nothing to woth I2C, too. It is for proper energy metering function. Usually it is in Megahertz range. It will be OFF on power down.

* the crystal of the MCU (as the I2C master) determins the speed of the I2C (and MCU setup).

So generally it is possible to run a whole system with just one crystal, indeed it has some benefits, but you need different frequencies and therefore you need clock divider and multiplier....this usually is more effort than just using different crystals.
And for an RTC the clock needs to run all the time...with a single crystal clock system it will draw too much power.

Klaus

- - - Updated - - -

Hi,

ADE7953 and MCU with a single 3.58Mhz crystal? I was thinking of connecting it in parallel.

Click to expand...

Be careful with this. A crytal is not a complete oscillator with logic level outputs.

So one device needs to act as the oscillator (master)...with enough output signal to drive the other device (slave).
The input impedance of the slave device may influence the operation of the master device.

In case of lenghty clock traces or low impedance inputs I recommend to use a clock buffer.

Klaus