Data transmission using SPI interface in nRF52832

Question

Hello everyone! 
 I have been working on getting data from ADS1299 ( ExG bio-signal sensing). I successfully got data from this IC using STM32F407VG (Cortex M4) and transmitted the data to PC via serial communication. I intend to use nRF52832 so that the data can be wirelessly transmitted. My question here is: 
 1) The ADS1299 chip is a 24-bit ADC chip with 8 channels. I want to collect the data from all 8 channels which means each data frame would has at least 24 bytes long (24*8), not to mention status bytes (3 bytes) and other bytes added for monitoring data integrity. In short, each data frame would has 30 bytes long. As far as I know, a package data is only up to 20 bytes for BLE protocol, thus do you have any suggestion in my application? 
 2) I suppose it is doable to send 30-byte package each time. To send the data to mobile app, I still need to partition them into 8-bit data each and use NUS service to send. As I make a quick calculation, it requires at least a baudrate of 150000 byte/second if sampling rate is 500 Hz. Will the nRF52832 be able to transmit data to mobile app with this baudrate? 
 P/s: SPI interface I am using is 4-wire interface. 
 Thank you!

Andy · Accepted Answer

I'm working on a project where I get ECG samples from an ADS1292, but I get only 500 samples from just one channel. 
 1) My approach to this problem was to put all the data in a ring buffer. I'm using the nrf_queue library to buffer the samples, and I start sending the data over notifications once I have buffered 500 samples (once a second basically). At this point I start sending notfications as fast as possible. There are loads of threads on how to achieve this by queueing as many notifications as possible and then reacting on BLE_GATTS_EVT_HVN_TX_COMPLETE events. 
 Your current approach won't ever work. You won't be able to send notifications each time you get a sample from the sensor, even if you lower the connection interval to the minimum (7.5 ms). You need to increase the ATT MTU size and make use of the data length extension feature to achieve this. Take a look at the throughput example if you need some pointers. 
 2) How did you come up with that number? You need to send (3 bytes/channel * 8 channels * 500 samples/second) = 12000 bytes/s, which is totally achievable. You might run into problems with iPhones though: in my experience they force the connection interval to 30 ms, but this is something that you need to test. 
 If you run into problems, start optimizing your data frames: do you really need to sample all 8 channels?

Data transmission using SPI interface in nRF52832

Top Replies