Verification for schematic for custom Board

[Moghees]

Hi,

We are design custom Board using nRF52840 SOC.

1. We are operating it in Normal mode, using internal DCDC converter
2. We have used configuration 5 from reference design
3. From reference design we have removed NFC pins connection and USB connection because we do not need it in our design
4. We will be programing our custom board using nRF52840 DK
5. We have attach led using load switch at P0.15
6. We have attach buzzer using load switch at P0.16
7. We have attach button at P0.24
8. We have taken antenna schematic from nRF52840DK hardware files and remove external connector. Have we done it correctly?
9. Have we connected reset button correctly?

I am attaching the Schematic design for our board can you verify it if it is correct or we are missing something, If it is unclear in picture I can also attach schematic file

Thanks in advance

Regards,

Moghees Bin Zahid

[Martin Sivertsen]

Hello Moghees,

I have a significant work load at the moment, but I'll get to your board soon, you'll hear from me tomorrow at the latest.

At the same time I have to mention that we rarely do full schematic reviews, we just don't have the resources for it, we focus on the nRF-device and RF front end (matching circuitry and antenna layout).
If the board is fairly simple we can sometimes look at the rest as a courtesy, but it just becomes too time consuming to do it for every customer design.

Best regards,

Martin S.

[Martin Sivertsen]

Hi Moghees,

Apologies for the delay.

Moghees said:

Are GPIOs mentioned above fine or should I use other ones?

Yes, they are fine.

Moghees said:

I am using the same Ferrit Bead for Battery as of USB-C, is it ok?

You don't need ferrite beads for the battery, but a SPDT switch would be a good feature to be able to disconnect power and power cycle the device in a simple way.

Moghees said:

As I am operating nRF52840 SoC on 3V, the ADC channel can only take a max of 3V as input, for that purpose I am using Zener Diode can you verify Battery Monitoring Circuit?

It would likely be easier (and cheaper) to just adjust the value of resistors R1 and R3 to get a Vout within the correct range.

Moghees said:

Now we have 2 power sources i.e, 5V from USB-C and 6V from the battery, for source selection I am using Schottky Barrier Diode Oring Scheme, can you verify its circuit?

It looks fine to me.

Moghees said:

I am using Linear voltage regulator HT7830 for getting 3V to run nRF52840 SoC.

It looks fine to me.

Moghees said:

I am exposing pins to connect the micro and Reed switch, is it ok?

Yes.

Moghees said:

Do you recommend adding capacitors with motor driver IC?

Yes, the datasheet of the motor driver IC usually has a recommended circuit or application circuit with recommended values. But a tantalum (polarized) capacitor of 10 uF and a ceramic one of 100 nF is probably not a bad idea.

Best regards,

Martin S.

[Moghees]

Thank you for the detailed reply Martin. Sorry for causing you inconvenience. 

Martin Sivertsen said:

You don't need ferrite beads for the battery, but a SPDT switch would be a good feature to be able to disconnect power and power cycle the device in a simple way.

I will remove ferrite beads but just to be clear I have a different connection for the battery and USB. SPDT will be only needed to connect and disconnect batteries? 

Martin Sivertsen said:

It would likely be easier (and cheaper) to just adjust the value of resistors R1 and R3 to get a Vout within the correct range

The problem is that 4XAA cells in series should theoretically give 6V but practically some cells can have voltage above than 1.5V which will result in voltage greater than 6, which will result in voltage across R3 greater than 3V and nRF52840 SoC can be damaged. One solution is that I use Zener other is that I assume the max voltage from the batteries can be 8V and calculate the value of R3 in that case voltage across R3 will not exceed 3V, So Should I use the 2nd approach or Zener is better?

Thank you so much for taking a timeout for this

Regards,
Moghees Bin Zahid

[Martin Sivertsen]

Hello Moghees,

Moghees said:

SPDT will be only needed to connect and disconnect batteries? 

Yes.

Regarding the battery measurement: Let's assume that the single cell alkaline batteries have a maximum voltage when fully charged of 1.8 V, four of them gives a total voltage of 7.2 V. Choosing R1 = 1M ohm, and R3 = 500k ohm on a standard voltage divider gives us a Vout of 2.4 V which is within range. Having such high values for R1 and R3 also has the added benefit of reducing current leakage. If you add a MOSFET in series and control the gate with a GPIO you can even turn the battery measurement on and off at will.

Best regards,

Martin S.

[Moghees]

Thank you for the reply. I have added a load switch to control battery monitoring and I will remove the Zener diode and recalculate values of R1 and R3

I have one question regarding the ADC channel of nRF52840

I was reading nRF52840 SoC documentation. I found the following table that shows the values of R1 for a particular sampling time

I have chosen a sampling time of 40us for which max resistance could be 800K, I am using value 787K for R1 and will calculate the value of R3

I want to ask if my understanding regarding ADC as mentioned above is correct?

Regards,
Moghees Bin Zahid