Why is there a maximum consecutive transmission time?

[Tim]

In section 17.1.9 of the nRF51 reference manual it states:

Maximum consecutive transmission time is defined as the longest time the RADIO can be active transmitting before it has to be disabled, i.e. the longest possible time between READY event and DISABLE task.

Maximum consecutive transmission time for the RADIO is 1 ms running of a 60 ppm crystal and 16 ms running of a 30 ppm crystal.

I have a few questions:

1. Why does this exist? (And why does it depend on the crystal?)

2. Can I just re-enable the radio immediately after it is disabled?

3. What happens if I go past the 1 or 16 ms time limit?

[ovrebekk]

Hi Tim

1. The radio in the nRF51 runs in a so called "open loop" mode. This means that the carrier frequency is calibrated using the 16MHz clock at the start of the packet, but after that it is left to drift to save power. If the TX runs too long in open loop mode it might drift so far that the receiver can not receive the incoming packet. With a more accurate crystal the initial frequency is more accurate, and it takes more time for the frequency to drift outside the limits.

2. Yes.

3. At some point the link will break because the TX and RX frequencies are too far apart.

Best regards Torbjørn

[Tim]

Makes perfect sense, thanks!

[ovrebekk]

Happy to help :)

[Scott]

What about the RX channel? I am seeing a disable every 1 ms while in Rx mode.

[ovrebekk]

There is no limit in RX mode, you can stay there as long as you like. If the RX is being disabled every 1ms it must be software controlled, unless it is the packet reception itself you are seeing.