Minimalistic schema

[David Celjuska]

I am looking for a minimalistic schema design using BLE for collecting data from i2c (twi) sensor and sending them over blue-tooth to the receiver. I need the following features:

1. blue-tooth data transfer
2. i2c support
3. reliable time measurement (knowing the time that passed since start in seconds). Precision +-3 minutes within 10 hours is fine. I would like to avoid any external crystal if possible.

Any suggestions for the minimalistic schema? Ideally i need it to fit on 7x10mm double-side PCB (the sensor is 4x4mm large). The blue-tooth distance range is not so important. If it will communicate in 5 cm distance it is okay (do I need any antenna?) I was thinking about nRF51822 but typical schema shown in data-sheet is still a bit complex (I believe it is possible to shrink number of components further).

Sorry for the novice questions, I am starting with that and need to know if it is feasible to design such a tiny PCBs with these chips before I dive into it.

Thanks

[Janek Mann]

Have a look at the nrf51822_ceaa_2-layer reference layout, I think it could be cut down to around 7x10mm. Of course you'd have to place your I2C device on the bottom side of the board so the routing may get a little tricky (not much space for the vias with a 2-layer board). It would probably be easier with a 4-layer board with blind vias, any reason you're restricting yourself to 2 layers? You do need the 16MHz crystal but can get away without the 32.768kHz crystal with your accuracy needs (250ppm of 10 hours is only 9 seconds). For 5cm you may get away with no antenna or you can try a wire antenna (preferably dangling but you can experiment with different foldings).

Don't forget about the battery; you may find it's not easy to find a battery that small that can handle the peak current demands of the radio. But there are some very small LiPo batteries around.