TWIS instances

RE: TWIS instances

awneil — Mon, 08 Jul 2019 09:01:33 GMT

[quote userid="2116" url="~/f/nordic-q-a/49410/twis-instances/197026"]as long as the total capacitance on the bus is not too high[/quote]

Indeed.

Anisa Note that this is a standard I2C thing - not specific to Nordic.

I2C is designed for short connections between chips on the same PCB - so, as ovrebekk says, capacitance should not normally be a problem.

But a common case where problems do often arise is when prototyping and attaching dev kits together with long wires. Solderless breadboards are particularly notorious for having high capacitance.

[quote userid="2116" url="~/f/nordic-q-a/49410/twis-instances/197026"]if you connect a scope to the bus you can see if the signalling looks OK[/quote]

Absolutely!

An oscilloscope is an indispensable tool here!

[quote userid="2116" url="~/f/nordic-q-a/49410/twis-instances/197026"]If capacitance issues do occur there are some ways to handle it[/quote]

there are also chips available for "extending" I2C buses where capacitance becomes a problem ...

[quote userid="2116" url="~/f/nordic-q-a/49410/twis-instances/197026"]I would suggest adding a single slave at a time and make sure it all runs nicely[/quote]

Absolutely!

As a general rule, always start simple - and work up...

RE: TWIS instances

ovrebekk — Mon, 08 Jul 2019 08:36:35 GMT

Hi Anita

In order to communicate to an I2C slave the nRF52 device needs to be in master mode.

In other words you should use a TWIM instance, not a TWIS instance.

A single TWIM instance should be enough to communicate with 4 slaves, as long as the total capacitance on the bus is not too high (in general the more slaves you have the more capacitance you get). Most likely this won't be an issue, but if you start to see communication issues when adding more slaves then it is usually caused by excessive bus capacitance. I would suggest adding a single slave at a time and make sure it all runs nicely (if you connect a scope to the bus you can see if the signalling looks OK).
If capacitance issues do occur there are some ways to handle it, such as adding stronger pull up resistors on the bus (lower resistance), setting the nRF52 pins in high drive mode, and reducing the I2C clock frequency.

When using the twi master driver you have to provide the address for every transaction, allowing you to select which sensor to talk to.

For an example, have a look at the API documentation for the nrf_drv_twi_tx(..) function, and note the address parameter.

Best regards
Torbjørn

RE: TWIS instances

awneil — Mon, 08 Jul 2019 08:24:03 GMT

Still not entirely clear, I'm afraid - a diagram is a far more effective way of conveying this sort of information!

In particular, is the nRF52 is the Master on the I2C bus?

[quote userid="81289" url="~/f/nordic-q-a/49410/twis-instances/196964"]They all have different I2C address[/quote]

Of course they do - that is a fundamental requirement of any I2C bus!

https://www.nxp.com/docs/en/user-guide/UM10204.pdf

https://en.wikipedia.org/wiki/I%C2%B2C

RE: TWIS instances

Anisa — Sun, 07 Jul 2019 10:54:29 GMT

May be I should explain further in detail. My 4 slaves are sharing the same I2C bus. They all have different I2C address. In this case, should I create 4 twis instances or only 1 twis instance?

I understood it should be 1 instance. How should I define I2C address of slaves, initialize and enable?

RE: TWIS instances

awneil — Fri, 05 Jul 2019 13:44:13 GMT

[quote userid="2116" url="~/f/nordic-q-a/49410/twis-instances/196856"]you can use a single master on the nRF52 device to communicate to all your slaves, since TWI allows a single bus to be shared between devices.[/quote]

Indeed:

https://infocenter.nordicsemi.com/topic/com.nordic.infocenter.nrf52832.ps.v1.1/twim.html#concept_scx_f5p_xr__fig_swz_gvp_xr

RE: TWIS instances

ovrebekk — Fri, 05 Jul 2019 13:22:35 GMT

Hi Anita

I am a bit confused whether or not you need the nRF52832 to be the TWI master or slave.

Do you really have 4 TWI master devices that all need to talk to the nRF52, or do you have 4 TWI slaves that you are trying to connect to the nRF52?

Normally your controller (ie the nRF52) would be the master on the bus, and your sensors would be slaves.
In this configuration you can use a single master on the nRF52 device to communicate to all your slaves, since TWI allows a single bus to be shared between devices.

Best regards
Torbjørn

RE: TWIS instances

awneil — Fri, 05 Jul 2019 08:49:05 GMT

Look in the Product Specification - it tells you how many TWIS hardware blocks are present in the chip. You can't have more than that!

But note that the Product Spec also says:

"The TWI slave is able to listen for up to two addresses at the same time"

So maybe you could have 2 instances each listening to 2 addresses ... ?