The 5340 takes a long time from power-on to code running

[GinoWang]

Hi,

  I recently found that the 5340 has a slow startup time on my custom nRF5340 based hardware, it takes about 3 seconds from power-on to the Main code running.

  I did the following experiment:

   1、on the DK development board  :    the same code took about 1 second from power-on to  the Main code running ;

   2、on my custom nRF5340 based hardware:     When a low level reset pin is set while the code is running, the code can run again within 1S;

This is our schematic:

   1、3V3_EN & nRF5340_RST is Controlled by another MCU(#2).  When power on,  the  #2MCU will set 3V3_EN enable and nRF5340_RST high impedance state & waiting for 5340 to start and receive the handshake message initiated by 5340 through the serial port

[Simonr]

Hi

Since the application has a pretty normal startup time on a DK, my guess is that the crystal oscillator(s) in your design are not loaded optimally and thus takes a while to stabilize for the application to use. Do you have the datasheets of the external crystals you're using and I can take a look to see what they should optimally be loaded with?

We generally use the following formula to find the best capacitor values (Ccap) using the load capacitance (CL) in the crystal datasheet.

Ccap = CL * 2 - 4

Best regards,

Simon

[GinoWang]

Hi

   In my custom nRF5340 based hardware: Ccap = 20pf   CL=12.5

I did the following test:

Case1:  CONFIG_SOC_ENABLE_LFXO=y、 CONFIG_SOC_LFXO_CAP_INT_6PF=y  、Ccap=15pf   :  startup time≈ 1.2Sec

Case2: CONFIG_SOC_ENABLE_LFXO=y、 CONFIG_SOC_LFXO_CAP_INT_6PF=y  、Ccap=20pf   :  startup time≈ 2.5Sec

Case3: CONFIG_SOC_ENABLE_LFXO=y、CONFIG_SOC_LFXO_CAP_EXTERNAL=y  、Ccap=20pf   :  startup time≈ 2.5Sec

Case4: CONFIG_SOC_ENABLE_LFXO=y、CONFIG_SOC_LFXO_CAP_INT_7PF=y  、Ccap  Null   :  startup time≈ 1.2Sec

If I continue to use Ccap=20pf, how should I set it to ensure that the boot time is no higher than 1.2 seconds?

By the way, we have produced a large number of machines with a 20pf configuration,It is not feasible to rework and replace 20pf.