ANT Forum

Hi all,

I have a CC2571 which talks to a Garmin Edge500 bike computer. I'm sending power values every 2 seconds to the Garmin. To save power is it possible to shutdown the CC2571 completely between transmissions?

Currently I initialise it once and simply send my values every 2 seconds, I see the transceiver seems to do some of it's own periodic transmission or reception every 250ms at the rate which I programmed it at. Does this stuff need to be happening in the background? I have a feeling it does, but maybe somebody can confirm.

Eoin      

Hi Eoin,

When the channel period is set to 4Hz (and the channel is configured as a master) the ANT chip will transmit a message every 4Hz. If you have not sent it a new message to transmit, then it will simply transmit the previous message again. This is what you are seeing every 250ms.

To be compliant with ANT+ device profile you do need to transmit at 4Hz. Updating your message every two seconds is also ok (but is unusually low for a bike power meter). So if you are going to be distributing this product, then you have to stick with the channel configuration specified in the profile. Distributing non-compliant product would violate the terms of use for the ANT+ network.

However if you are ONLY building this sensor for your own use, then you could try reducing the channel period by a factor of two. This would appear to the edge like bad interference knocking out every other message that it expects to receive. It will increase the time it takes for the Edge to find and connect to your sensor. It will also make the system more prone to drop-outs and less tolerant of interference - but may still function well enough for your project.

How come your power consumption is so critical? Have you checked that the current consumption on your device matches the expected values? The power estimator is here: http://www.thisisant.com/developer/components/power-estimator/

Hope that helps,

Kat      

For me the power is calculated and averaged/ integrated over one entire pedal stroke. Therefore if the cadence is 30RPM the power value only needs updating every 2 seconds. Within that 2 second period if the rider has been pushing hard or soft on the pedals, it won't matter because the overall effort over the entire cycle will be reflected in the averaged value. This is why I only need to send that often, not sure if this approach suits the ANT+ protocol.

As the RPM increases I will indeed need to be sending more often. 4 times a second for 240RPM, and twice a second for 120RPM.

The real reason why I'm trying to decrease the radio activity is a weird nasty knock-on effect it's having on my power measurement, the EMI caused by the 4 Hz transmission is saturating my low power electronics for power measurement. Looks like I'll have to somehow find a work around for that issue...

The information about the message being repeated in the background is useful, wasn't aware of that. So you are saying if I don't send every 250ms it will re-send the message for me? If I sent every 250ms myself, could I be sure the CC2571 won't do any more of its own transmissions? (This would fix my EMI issue)      

OK - you're an 'event-synchronous' power meter (to use the profile terminology) - that's a totally normal approach.

The ANT message protocol and usage doc describes how the channel operates. Yes, it will resend the message for you. You can use the EVENT_TX messages received from ANT to know when it has sent. You should ideally use these to make sure you write new data to the buffer at the right time. One message is sent each channel period (each 250ms): if you've provided new data, then that data will be sent. If you haven't then the old data will be sent.

You might need a hardware fix to decouple your circuits - if you need support with that then you should contact TI