topic Re: S12DP512 interface with 3.3V devices. in Sensors

S12DP512 interface with 3.3V devices.

monstor — Mon, 04 May 2015 10:15:46 GMT

Dear all,

I want to use S12DP512 to interface with a pressure sensor and accelerometer.

The voltage of these sensors Vdd are 3.3V and the Vdd of S12DP512 is 5V.

How can the MCU interface with these sensors? Should I need a level shifter? or They are TTL compatible?

Thanks for your helping.

Re: S12DP512 interface with 3.3V devices.

TomasVaverka — Mon, 04 May 2015 11:37:02 GMT

Hi Henry,

You did not specify which sensors you are going to use, but in general I would suggest using both a logic level converter and a 3.3V regulator to allow everything to run at the proper voltages.

Regards,

Tomas

PS: If my answer helps to solve your question, please mark it as "Correct" or “Helpful”. Thank you.

Re: S12DP512 interface with 3.3V devices.

monstor — Tue, 05 May 2015 01:03:00 GMT

Dear Tomas,

I would like to connect to 3 I2C sensors. 1 of them is 5.5Vdc max. so

there is no problem of it. The others are 3.6Vdc max.

They are :

1. Freescale FXLS8471Q (Linear Accelerometer)

2. Measurement Specialties MS5607-02BA03 (Barometric Pressure Sensor)

I cannot confirm are they TTL compatible. If yes, I think I don't need a

level shifter IC. Do you have any recommendation?

Really thanks for your helping.

Best regards,

Henry

Re: Re: S12DP512 interface with 3.3V devices.

TomasVaverka — Tue, 05 May 2015 10:39:34 GMT

Hi Henry,

Pulling the bus to 5V is not really a solution since both the SDA and SCL pins are not tolerant of voltages higher than VDDIO + 0.3V (3.6V max).

However, looking at the MC9S12DP512 specs, the minimum voltage for a high level is 0.65*Vdd, which is 3.25V at a 5V supply. So, if you pull the I2C bus to 3.3V, you might not need an I2C level shifter.

Regards,

Tomas

Re: S12DP512 interface with 3.3V devices.

monstor — Tue, 05 May 2015 10:53:25 GMT

Dear Tomas,

Thanks for your reply.

I can understand your theory. That mean MCU side can accept the high

signal from sensor once the voltage level is higher than 3.25V.

However, can the sensor accept the high signal from the MCU? The VDD of

the sensor is 3.6V max.

Best regards,

Henry

Re: S12DP512 interface with 3.3V devices.

TomasVaverka — Tue, 05 May 2015 11:45:14 GMT

Henry,

I do not quite understand what you mean by „can the sensor accept the high signal from the MCU“. The trick is to connect the pull-up resistors to the lower of the two voltages, in our case to 3.3V. When the output transistor is off, the resistor connects the pin to 3.3V (not to 5V).

Regards,

Tomas