programing and interrupts on custom PCB using nrf51822

Question

Hey, 
 I am designing a custom PCB with the nrf51822. I would like to use the reference layout that Nordic provides, specifically nrf51x22_qfaa. My project involves reading an accelerometer and gyroscope via TWI and transmitting certain measurements via BLE. I am designing this PCB in Altium Designer and I am using the reference files Nordic provides and footprints from their libraries. 
 I'd really appreciate it if any of you could help me clear up a few things: 
 1.) To program the chip on my custom PCB I am planning on including in my design the same 10 pin SWD header Nordic uses for the development boards with J-Link. Currently I am connecting SWDIO and SWCLK directly to the chip with a 12k pulldown resistor on SWCLK. Is this ideal? I'm confused about nrf_RST. 
 2.) When I connect my accelerometer and gyroscope to the nrf51822 via TWI can I use any GPIO pins I want? I am planning on connecting SCLK and SDA to GPIO pins 8 and 9 and then connecting interrupts to GPIO pins 10-13. I also need to be able to wakeup on interrupts. (I think I remember seeing something in the user guide about restrictions on these pins when Softdevice is enabled but cant find it). Also I am planning on not including any external resistors and just using the internal pull ups. Is this advisable? 
 Are there any other suggestions you have given the nature of my project? I'm a student and this is the first PCB I've designed. 
 Thanks for the help!

mnhs · Accepted Answer

Hi, 
 
 
 Yes, you can implement the programming interface like that. I was playing with the interface and if you accidentally forgot to put it there (like me :-) ), it works even without the 12k pull-down on SWCLK. But let it be there. The NRST pin is "integrated" into SWDIO pin. Thus, unlike for example the STM32 family, you need only 4 pins to program the device, not 5. 
 
 
 Yes, you can use any pin for the interface. There is just a restriction on ADC - there is a specific range of pins as stated in datasheet. As for wakeup interrupts, I am not completely sure - try to find a corresponding page in datasheet and wait for advice from anyone else here. The internal pull-ups are fine, if the traces are short and do not include any excessive capacitance. Otherwise, you should experiment with external resistors to achieve optimum rise and fall shapes. 
 
 
 PCB layout considerations - there is a lot of things to look after to make a good design. Just a short must-do list. Do not forget to make proper decoupling, several caps as close as possible to the power lines of the chip. A combination of several 1nF, 22nF, 100nF will make the job. Do not place any tracks under the balun and the 50 Ohm impedance matched trace to the antenna. There should be a clear ground plane line under the RF circuitry. If the path to the antenna is long, you must match the path to 50 Ohm. There are several calculators called "microstrip impedance calculator" or something like that on the Internet. Otherwise, you will lose some output power there and reduce the range. The power line should be stable, without any significant ripples. If not using 4-layer design and nice power plane, make the power traces wider than signal traces and make them as straight as possible, without many vias. Implement ESD protective devices to all connectors. Use low capacitance 32.768kHz and 16Mhz crystals and place them close to the chip. 
 There are a lot of other tips and guidelines. Read some guides. This is a document from Texas Instruments about analog-digital circuits. 
 www.ti.com/.../sloa089.pdf 
 Or attend any PCB design orientated course. The PCB design can be a bit alchemy, especially if you are going over 100 Mhz or you are measuring very small signals and every detail matters. And well, RF design is a true magic :-).

programing and interrupts on custom PCB using nrf51822

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