Undocumented TX - RX radio delay

[Fmag]

Hello,

I am using the nRF52840 DK (same applies also for the dongle). The setup is quite simple. I have two nodes on my desk, one transmitting packets and the other receiving. Via PPI, I connect some of the radio events, such as FRAMESTART, END (in 802.15.4 mode), or ADDRESS, PAYLOAD (in BLE modes), to GPIO pins and observe them with a logic analyzer. I can see a constant delay for the same events between transmitter and receiver. This delay depends on the data rate and is about 10 bit times in each mode. For example, in 802.15.4 mode (250kbps) the delay between the FRAMESTART (or END) events between transmitter and receiver is always around 40 us. I have the impression that the delay is evenly distributed between the receive and transmit paths, without being sure. Can someone explain this delay to me? Are there buffers in the radio which cause this delay or does this somehow relate to the EasyDMA, which the radio uses?

Thanks in advance.

[ovrebekk]

Hi again

Apparently your findings are normal, but since we don't characterize or document this delay it is limited how much information there is to share. 

Most of the data processing on the receive side is clocked at the same speed as the RF bitrate, and as such the processing delay will to a large extent scale with the bitrate as you have discovered. There is also some constant delay that doesn't vary with the bitrate, but the magnitude of this delay is significantly smaller. 

Best regards
Torbjørn

[Fmag]

Hi,

thanks for your answer. Based on my findings I had the impression that the delay is distributed between transmitter and receiver paths but based on what you write it sounds more like the delay is completely on the receiver side. Do I understand this correctly? Can you quantify the constant delay, even if it is much smaller?

Cheers

[ovrebekk]

Hi 

The difference you measure in timing between the events on the TX and RX side should be the RX delay only, that is correct. 

Fmag said:

Can you quantify the constant delay, even if it is much smaller?

Somewhere around 500-1000ns I believe. 

Best regards
Torbjørn