Oscillator sharing on STM32F4 board & SMBus doubts

[harinath]

Posted on September 10, 2015 at 07:48

Hello Dears,

We would like to share the 32.768 oscillator with STM32F405 mcu and with LPO pin of ST Bluetooth module SPBT2632C2A. 

Is it possible to connect the OSC32_IN or OSC32_OUT pin to LPO pin of bluetooth module ?

Also one more question. In our design, we want to use VL6180x sensor (It has I2C).

But our STM32F4 mcu doesn't have empty I2C peripheral. Can we use SMBus to communicate with VL6180x ?

Thank you 

#smbus #oscillator-sharing

[waclawek.jan]

Posted on September 10, 2015 at 10:12

> Is it possible to connect the OSC32_IN or OSC32_OUT pin to LPO pin of bluetooth module ?

I wouldn't do that. Oscillator circuits are low power and notoriously highly sensitive to additional loads.

You can direct LSE output to MCO1 (not in VBAT domain, though).

> Also one more question. In our design, we want to use VL6180x sensor (It has I2C).

> But our STM32F4 mcu doesn't have empty I2C peripheral. Can we use SMBus to communicate with VL6180x ?

You mean, to share one bus with a SMBus peripheral? I don't know - probably yes but it might fall over on details, I am not familiar with SMBus. It's easy to bit-bang an I2C master, though, if you have a spare timer and can tolerate the additional software load.

JW

[harinath]

Posted on September 11, 2015 at 06:31

Good to know LSE can be redirected to MCO1. I considered this.

In my case we have a Li-Ion battery connected to board, so we connected 3.3v to VBAT pin. In this case, still I we can redirect the LSE onto MCO1 ?

Here is my understanding, if we connect RTC battery to VBAT, then it's not possible to redirect LSE output to the MCO1.

Regarding SMBus, The following helped me to understand the major differences.

http://www.ti.com/lit/an/sloa132/sloa132.pdf

But I'm afraid to use SMBus host to control I2C slave device ( though it is possible at the cost of time) . I don't see people using this way, so I'm worried.

Thank you