Approach for synchronize two devices

[Michael01101]

Hello,

in our project we have a nRF52840DK as central (s140) and a custom board with nRF52832 chip as peripheral (s132). The software on the nRF52840DK is based on the ble_app_uart_c example and the software on the nRF52832 chip is based on the ble_app_uart example. The nRF52832 sends data to the nRF52840DK via NUS. This works fine. 

We want to roughly know, how "old" our data is, when it is received by the nRF52840DK. For this the data get timestamped. The idea is that the real-time clocks on the two devices are roughly in sync. The nRF52840DK will then know how "late" the data is. After some research, this should be achievable by implementing the following:

• Device 1 enables the NUS notification and starts the real-time clock with the right prescaler.
• Device 1 makes use of the ble_radio_notification to know exactly when to start the RTC.
• Device 2 receives the "enable of the NUS notification" and starts the real-time clock as well with the same prescaler.

For implementing this correctly, we have a few questions:

1. Where gets the NUS notification enabled in the ble_app_uart and ble_app_uart_c example?
2. How to use the ble_radio_notification right?
3. Where gets the "NUS notification enabled" received by ble_app_uart and ble_app_uart_c example on the other device?
4. How to start the real-time clock with the right pre-scaler?

Thank you so much in advance.

[haakonsh]

We want to roughly know, how "old" our data is

 Please define roughly first.

[Michael01101]

Thank you all for your answers.

Sorry for this inaccurate statement.

Roughly: +/- 2ms

Thank you in advance.

[awneil]

so how about the idea of just setting a sync reference in the log, and working from that?

[Dmitry]

The minimal connection interval for BLE is 7.5ms, so 2ms is a challenge. If you really need an absolute time, you can try this way:
- get in sync with your connection - for example, using radio notifications or a new sd_ble_gap_conn_evt_trigger_start API call. Knowing the time of last event and connection interval, you can calculate the time of next event
- ensure that no packets are in hvn queue (stop any activity for some time)
- send a sync message with timestamp of the next connection event plus an estimated time of transmitting and handling of this message
- if BLE_GATTS_EVT_HVN_TX_COMPLETE is received right after the next connection event (no lost packets), send a confirmation message - other side can trust this timestamp, otherwise try again.

It may sound a bit complicated (until you take a look at RFC1305  ) Another way is this method of time sync, if you're not limited only with BLE protocol.