Thingy:52 Power Concept

RE: Thingy:52 Power Concept

MartinBL — Fri, 02 Feb 2018 13:07:57 GMT

Unless you have other devices connected to the same SPI bus, I don't see any reason to use a switch. If you disable the nRF52's SPI when you power down the MPU, you shouldn't need to worry about current consumption either.

RE: Thingy:52 Power Concept

ZAC — Fri, 02 Feb 2018 07:19:03 GMT

Good (and nice new webpage for devzone!). Do you think that this implementation would bring any benefit in case of an SPI implementation or one can live in that case with a switch for cutting-OFF VDD_MPU only?

RE: Thingy:52 Power Concept

MartinBL — Fri, 26 Jan 2018 13:49:56 GMT

I talked to the guy who designed the Thingy.

The double throw switch is there because there are multiple slaves on the same I2C bus. If you turn off one of them it is likely that it will mess with the bus. The reason why a NX3DV2567 double-throw switch is used instead of a possibly smaller single throw is simply because the NX3DV2567 was the cheapest option at the time.

RE: Thingy:52 Power Concept

ZAC — Fri, 26 Jan 2018 11:37:59 GMT

I think, if the VDD_MPU pin alone is cut (i.e. a single-pole switch is used), the SDA and SCL will still be connected to VDD through the R22 and R23. So the background why 4-pole switch must have to do with leakage current and therefore energy saving?

RE: Thingy:52 Power Concept

ZAC — Fri, 26 Jan 2018 11:32:43 GMT

Thanks for the reply. I have added pics for that. Indeed pulls-ups are there. So should I assume it is not for power saving? Did not find anything about pull-down to GND in the MPU's datasheet: The MPU-9250 always operates as a slave device when communicating to the system processor, which thus acts as the master. SDA and SCL lines typically need pull-up resistors to VDD. The maximum bus speed is 400 kHz.

RE: Thingy:52 Power Concept

MartinBL — Fri, 26 Jan 2018 11:01:12 GMT

Hi,

I couldn't find anything about what kind of state the I2C pins are in when the MPU is powered down. This is probably a question for Invensense. If they are high impedance though, the pull up resistors on the bus should pull the lines high and it shouldn't matter. If the MPU of some reason pulls the line down to GND you will have a current flowing through the pull ups. There seems to be some 4.7kOhm pull ups on the bus and the TWI drivers probably also enables the internal 13kOhm pull up resistors.

Electrical spec for the nRF52's SPI can be found here.

Hope this helps. Not sure if I quite understood your situation.