Drive i2c device with LPC8N04

[pmg]

Can I talk to a peripheral device using i2c with the LPC8N04 when it's powered by NFC?

May I power a peripheral device from the LPC8N04?

What is the output voltage and current supplied by the LPC8N04 when powered via NFC?

[Sabina_Bruce]

Hello!

   What you stated is correct the energy harvesting is meant to only power the LPC8N04. However as you mentioned in your 2nd point a common practice is to use the high-current driver GPIO pins with a capacitor to maintain a steady flow of energy for an external peripheral. These high-current drivers allow for a maximum of 20 mA of current, but this is not something that can be guaranteed, due to the design of the antenna on both the reader and the LPC8N04, as well as the distance between them. The steps that you have taken are exactly what we would recommend to maximize the ability to produce a high enough current to power an external device.

The figures that quantify this energy harvesting are not available. So I will go ahead and check with the design team, to see if they can provide this information. In addition to checking, why you are not able to communicate via I2C when powered by NFC(through LA & LB), I'll let you know as soon as I get an answer.
Sabina

[IvanRuiz · Accepted solution]

Hi pmg,

Covering the missing points, the internal team let us know that with a smartphone antenna, we can get at least ~10 mA which is directly related to the antenna design and distance.

The power provided by the RF field is mainly meant to power the LPC8N04 and allow it to communicate with an externally powered device through e.g. I2C; please connect the Pull-ups to the external device voltage domain and do not forget to connect GND in common.

Also remember that the system will be running at 2 MHz and the I2C bus will be operating in normal mode when powered from the NFC field.

Hope this helps!

Regards,

Ivan.

[Sabina_Bruce]

Hello!

To answer your first question you are able to communicate through I2C when powered by NFC. Keep in mind that in order to run the I2C you need 2MHz system clock but to run in fast mode you need 8 MHz. When running without a battery, energy harvesting is limited to 2 MHz system clock.

The LPC8N04 has the capability of harvesting energy, to power the LPC8N04. In this device there is not a pin that could power an external device, in addition the I2C lines are not capable of carrying enough energy to power an external device. The maximum current that is passing through these lines is 30mA when powered by an internal battery of 3.6v. 

Here is the datasheet for additional reference:

https://www.nxp.com/docs/en/data-sheet/LPC8N04.pdf 

Hope this helps!

Sabina

[pmg]

Hi!

Thank you for the helpful reply!

Since posting we've done some more debugging:

1) We are able to get the LPC8N04 talking over I2C to another chip when powered via USB. (From this we conclude it's not an I2C communication issue)
2) We are NOT able to get the LPC8N04 talking over I2C to another chip when powered via NFC (through LA and LB) and the NFC core/PMU - even with setting the clock to 2Mhz and disabling eeprom. The external I2C slave chip is powered via two of the four high drive gpio lines from the LPC8N04.

3) We ARE able to get the LPC8N04 talking over I2C to another chip when powered with an external NFC power harvesting chip (The NTAG I2C plus NT3H2111) - from this we conclude it's not a limitation of how much power NFC can supply from the phone generally.

We've heard off-handedly the LPC8N04 is just a combination of the NT3H2111 packaged together with the LPC804 - the datasheets don't confirm or refute that but logically it makes sense.

While the datasheet for the NT3H2111 does say how much energy it can harvest (up to 15mW but typically 5mA at 2V) - the datasheet for the LPC8N04 does NOT characterize the energy harvesting performance. As you noted it is kind of vague on that point saying "The LPC8N04 has the capability of harvesting energy, to power the LPC8N04" - by this am I to infer the energy harvesting is *only* for powering the LPC8N04 and *no* external accessories? I've read the datasheet 4 times and the only approximate characterization is on page 28 which describes the internal power domain:

1) Are there figures available quantifying how much energy the LPC8N04 can harvest? (This seems important to have in a datasheet for a chip which advertises it supports energy harvesting!).

2) Aside from reducing the clock, depowering the eeprom, and setting the GPIO to CDrive Fixed Voltage Mode and DDrive High Drive mode - are there any other steps we can take to maximize the LPC8N04's ability to power an external device via NFC?