NRF51822 : is that a normal power consumption

[julienh]

Hi,

I would like to have a confirmation of my interpretation of the consumption information that I found in the NRF51822 product spec to estimate the power consumption of my application.

My App : Use an RTC (1s timeout) to keep a record of the time after a first init with current time service and a smartphone. Switch ON-OFF LED if the time is between a specified slot.

When we are not connected, the device is in advertising, the PWM module is ON (update of the value each second). I use SYSTEM ON and scheduler (for interruption) in the main function.

Advertising interval : 1200ms Transmission power : 0dBm DC/DC disabled Processing (execution time is not measured) : Checking current time with time slot each second and update the PWM value.

Ressources : RTC / PWM / BLE

According to the NRF51822_PS, I thought that I HFCLK and LFCLK are always on and 1.2V is enable when CPU and BLE are enable. Is that right?

So :

• ITX,0dBm -> 10,5mA for Tx (advertising)
• Icpu -> 4mA
• What is the value for System-ON with RTC/PWM ?

What is the best power consumption that I can expect with this configuration?

May be, I will reduce the RTC update from 1s to 10s to decrease the CPU use.

Thank you

[Stian Røed Hafskjold]

PWM
Since the nRF51 doesn't have a PWM module, the PWM library uses a TIMER to toggle a GPIO according to the PWM frequency. The TIMER module requests the HFCLK. Since you are using BLE/SoftDevice, you already have a HF crystal on your board. So then the PWM timer will be using the HF crystal.

Run currents for PWM (timer and 16mHz crystal):

I_TIMER + I_X16M = 30uA + 470uA = 500uA

BLE
The base current when running the SoftDevice consists of System ON base current, LFCLK run current and RTC run current. Then you have the radio and CPU usage every adv/con interval, this varies with the adv/con parameters you specify. The numbers below are based advertising with 0dBm TX power and full payload (31 byte)

Base current:
I_ON,32k + I_X32k + I_RTC = 3.8uA + 0.7uA + 0.1uA = 4.6uA

Total charge for one advertising event, 0dBm, 31 byte (includes radio and CPU):
27uC

1200 ms advertising + base current gives an average current of:
27uC / 1200 ms + 4.6uA = 27,1 uA

So to sum up, average current during advertising with PWM library running is 500uA + 27uA

When not using PWM, the HFCLK will only be active during the BLE event (about 4 ms), and will reduce the average current significantly.

I would say that the CPU time used to update the PWM is so small that the current is negligible compared to the other currents.

[julienh]

Thank you for this theorical consumption study.

In my case, the PWM is updated each second and is always on so I will have 500uA power consumption in continue right?

[Stian Røed Hafskjold]

Yes, that is correct. You can reduce the consumption a bit by choosing a PWM frequency that allows the TIMER module to use the 1MHz peripheral clock instead.