Notification timing not correct.

[T IJ]

Hello Devzone,

I've been working on a system to set notifications on time ready for transmission.

The client sends a WWR package to my peripheral and I want my peripheral to send a notification back in the same connection interval.

The client and peripheral both have a 7.5ms min and max connection interval.

To my knowledge a notification has to be set before the transmission moment occurs.

My approach is to have one free running timer captured as soon as I get the WWR message, I'll add that captured value with x (roughly 6ms in timer ticks) to a new compare register to create an event to set a notification ready for the next connection interval.

When the compare event strikes I do a sd_ble_gatts_hvx(....) and set my notification ready.

I've added a picture to help understand what I'm trying to achieve. 

The problem I'm having is that the notification is not send as I pictured here.

Instead the notification is set at the right time but instead of being transmitted in the following connection interval, it is being transmitted in the connection interval after the intended interval.

I've tried changing the time for when the notification is set ready for transmission but this either resulted in ERROR_NO_RESOURCES or in the same behavior.

When I change the WWR and notification time so it is transmitted every 15ms instead of 7.5ms I get the following behavior.

This is really bugging me and I cannot find what I'm doing wrong here.  

Can anyone help me with my struggles?

Kind regards

[T IJ]

I haven't checked out the radio notification module yet. 

I have a capture in almost every state between receiving the WWR, transmitting the notification and setting the notification ready for the next interval and all are within the connection interval time.

WWR triggered at 0ms
Notification sent at ~600us after

Notification set at 6ms after the WWR.

WWR triggered at 7.5ms

no notification sent here.

Notification set at 13.5ms

....

The strangest thing I've seen is that if I keep the peripheral side the same and do not send WWR's it does send notifications every 7.5ms

[T IJ]

Ive made an Excel sheet with some information about the timing if that helps.

[T IJ]

After doing some more testing I've found out that the HVX complete event is set after the connection interval. Please correct me if I'm wrong.

By trying to set a new notification before this event happened I probably got the message NO_RESOURCES. So does this mean I am unable to send a WWR + notification every connection interval?

[Edvin]

You don't need to wait for a notification to send a WWR. But of course, if your WWR depends on the content of the notification, you need to do so. As I mentioned in the beginning of this post, I tested this using the ble_app_uart and ble_app_uart_c together, and then I send some data (WWR) in the notification event directly. This was sent the very next connection interval. Have you tried this?

[T IJ]

I do have tried this and I've seen this happen, but I don't know if this app is scalable to multiple peripherals.

I'm using a timer to send out WWR's at a fixed interval.

My WWR contains data that must be set as late as possible, this I am getting to work, its just that on the peripheral side things are not working correctly.