What is the startup time of a discrete nRF51822 module vs nRF51-DK board?

[seajayshore]

I know this is somewhat "how long is a piece of string". I'm happy for guidance rather than solid numbers:

I'm trying to spec a "keep alive" circuit for an nRF51822 project. The general principle is there is an LDO (with enable pin) that is first activated by a user pushbutton. Next, as fast as possible, the nRF51822 turns-on (now that it has power) and switches on a GPIO to keep the LDO enable pin high (therefore keeping the LDO on). This means the nRF can turn everything off by setting the enable pin to low.

To keep the enable line high during the time between the button being pressed & released, and the nRF51822 to wake-up, I'm just using a capacitor and resistor specified with the right discharge duration. So the time for RC-discharge to minimum enable-pin voltage needs to be a bit longer than the power-on time for the nRF51822.

I've looked at the product specification and especially at this related question here: devzone.nordicsemi.com/.../

I know that the practical start-up time (from power-input to GPIO-output) depends on the input voltage, I'm using 3.3V which should give pretty fast start-up (~2ms according to linked question).

But... Using the nRF51-DK dev. board to prototype this I see with my oscilloscope that the practical start-up time is more like 350-400ms... It's dramatically larger than what I was expecting... Obviously this start-up time includes LDO start-up, but that is ~50uS according to my scope.

So, my main question(s):

• Does the nRF51-DK take much longer to start than a raw/lone nRF51822 module?
• Approximately what start-up times could I expect with a real nRF51822 module? (Are values in the product spec. & the related question realistic for the power-on->GPIO-on start-up time I'm describing?)

My circuit is very flexible and I can manage even these large start-up times, but reducing it to product spec. values would be very useful to avoid a big capacitor.

Sorry for the long question, lots of detail. Any help/advice is appreciated.

[Øyvind Karlsen]

Hi,

The startup you're seeing is the low frequency clock which has a typical startup time of 0.3s. Please see the below logic analyzer trace where channel 0 is the toggled GPIO and channel 1 is the power. First with initialization of the LF-clock. (300 ms measured)

Then without initialization of the LF-clock. (0.8 ms measured)

For your application you can toggle the GPIO before starting the LF-clock. Or if you have a particularly need for clocking right at the start you can use the RC clock, which has a startup time of ~400 µs, before initializing your LF crystal later.

Best regards,

Øyvind

[seajayshore]

The voltage drop at the end of scope is due to the battery supply being ~4V that connects briefly to the enable pin, via the button which is also connected to a GPIO of the nRF51. I stupidly forgot to put some protection on this GPIO when I took this photo... Oddly the blue line, that responds with a higher-than-expected voltage exactly in sync with this higher supply voltage, is not connected to the enable pin. It's the response GPIO I'm using to test start-up times... Is there a chance that applying this too-high voltage to the other GPIO pin has affected how the other GPIO pins work? Or the nRF51 as a whole? The ~4V is obviously greater than recommended VDD+0.3V max. rating (so I'm correcting this in any case).