Using as much as 90 x BMP280 Digital Pressure Sensor on the same SPI busses..

[Arcenson]

I need technical help about the use of BMP280 Digital Pressure Sensor.
My project should use up to 90 of these SPI devices on a common Microchip PIC16F1789 SPI bus.
Yes 90 of this sensor. The 90 Chip selects will be generated by 8 x 74HC154 multiplexers
to accomodate the 90 needed CS pulses. Ok.
My concerns are about the MOSI, MISO, SCK data loading.
The BMP280 shows internal pullup resistances down to 70k ohms
and 400pf capacitance loading for each BMP280.
That means, for 90 paralleled devices, around 780ohms
and .036ufd on each MOSI and SCK busses !!!
No problem with the readings of MISO (BMP280 tristated SDO) all paralleled pins,
But I think the MOSI & SCK driving lines would need some kind of buffering
to keep the data reliable to each BMP280 right ?

[BSTRobin]

Hello Arcenson,

BMP280 is SPI slave device. If you use 90 SPI slave devices in same SPI bus, you should consider bus driver capability from master to slave. MOSI and SCK were sent from master side.

[Arcenson]

>BMP280 is SPI slave device. If you use 90 SPI slave devices in same SPI bus, you should consider bus driver capability from master to slave. MOSI and SCK >were sent from master side.

Ok that makes sense.. Something like 74HC541A  bus driver ?

But about the MISO side (the bus coming from all the 90 devices):
The BMP280 shows internal pullup resistances down to 70k ohms
and 400pf capacitance loading for each BMP280.
That means, for 90 paralleled devices, around 780ohms
and .036ufd on each MOSI and SCK busses.

Even tristated at idle state 90 devices would also bring
the MISO buss up to 0.036uf too ?
This would make each device reading hard to do ?

[BSTRobin · Accepted solution]

Hello Arcenson,

As we know, the decoupling capacitor which connect with power supply have an effective decoupling radius(It's the reason we need to put the decoupling capacitor around the sensor, the closer, the better). If the decoupling capacitor distance from the sensor is too long, the capacitor will NO effect on the sensor. According to this description, I don't think the capacitance of 90 paralleled device can arrive 0.035uf(same question for resistant).