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why CC2540-MiniDK board has ground plane on top and bottom layer?

chencan fan
Prodigy 150 points
Other Parts Discussed in Thread: CC2540, CC2530

Hi everyone. Our team is facing a problem about CC2540 hardware.

We have designed a two layer CC2540 board based on MiniDK, but Packet Sniffer received no advertising packet, it's OK while using  a development board to send advertising packets. Is CC2540 two layer design impossible to work? We found no hints about two layer design all over the Internet.

So we have to turn to 4 layer design, using CAM350 open CC2540 MiniDK manufacture file, we found ground plane on top and bottom layer. For a 4 layer board, usually the stack up is signal - GND - VCC - signal, there is already full GND layer below top layer, why  pour GND copper on top and bottom?

Much thanks for your reply.

 

  • 0
    TI__Mastermind 23440 points

    Hi chencan,

    It is possible to design a working two-layer cc2540 design - I have tried one. However, the performance with regards to EMC and antenna range will degrade.

    The reason for pouring gnd on routing layers is technical and i can't answer fully, but reducing EMI, e.g. crosstalk is one reason.

    If you have the board powered and are able to power up and program the chip, the problem can lie anywhere between the software and the antenna. If you are using one of the keyfob demo projects, they are not set up to advertise on power-up, but require a button press before advertising. This can be changed for example in SimpleBLEPeripheral by changing DEFAULT_DISCOVERABLE_MODE on line 98 to GAP_ADTYPE_FLAGS_GENERAL and initial_advertising_enable to TRUE on line 282 in simpleBLEperipheral.c.

    I hope this helps,
    Aslak

  • 0 in reply to Aslak N.
    Prodigy 150 points

    Hi Aslak,

    Thanks for your kindness. This afternoon we did a test on 4 layer CC2540 board designed by ourself, but USB dongle received error advertising packet. Can I send my 4 layer CC2540 board to your email box for some helpful suggestion? The matching network is based on CC2530 for their same impedance. We feilt really discouraged struggling for RF layout.

    Our RF line width is 8mil, the board thickness is 1.6mm, layer stack is signal --> VCC --> Multi-Ground (RF ground and CPU ground are divided) --> signal.

  • 0 in reply to chencan fan
    TI__Mastermind 23440 points

    Hi chencan,

    We recommend simply copying our reference design as far as possible.

    Regarding stackup, do you have GND directly below the RF signal traces? This is important.

    For calculating impedance, you can use a tool such as http://www.eeweb.com/toolbox/microstrip-impedance/. For instance, board thickness isn't very revealing or important compared to substrate height (sig to GND), trace thickness et al.

    BR,
    Aslak