Some questions on nRF52811 documentation

[Dmitry]

I'm studying nRF52811 manual, there are moments not clear to me:

1. Is I/Q sampling paused during SWITCHING slots? For example, if I configure 16 1-msec slots (8 switching + 8 sampling) with 125us sampling period, I'll get 64 or 128 samples?
2. TSAMPLEOFFSET is described as "Signed value offset before starting sampling in number of 16M cycles relative to the beginning of the REFERENCE state - 12 us after switching start". I'm completely lost... maybe you could describe this in other words to help me understand?
3. Can I use energy detection feature just for energy detection for custom 1-MBit protocol, not 802.15.4?
4. There is a note in radio electrical specification: "Typical sensitivity applies when ADDR0 is used for receiver address correlation. When ADDR[1...7] are used for receiver address correlation, the typical sensitivity for this mode is degraded by 3 dB". Is this because 1..7 are correlated sequentially? if so, can I be sure that sensitivity won't drop when I fill addresses 1..7 with the same value (I need only two addresses)?

Thanks

[Simonr]

Hi Dmitry

Sorry for the late reply, but it took some digging and asking around to find all the answers.

1. There is no IQ sampling during switching slots.

2. TSAMPLEOFFSET is one of the two start offsets for direction finding, and is the offset value in number of 16MHz clock cycles after the reference period (see figure 71 in product specification).

3. Energy detection is featured in the IEEE 802.15.4 operation, yes.

4. There is a correlator which can only operate on ADDR0.  It cannot work on 8 addresses in parallel.  Therefore only ADDR0 will be demodulated with a 3dB gain while the others will not.  Putting addresses in ADDR[1..7] does not degrade ADDR0 sensitivity. So sensitivity shouldn't drop when only filling it with 2 addresses.

Best regards,

Simon

[Dmitry]

Thanks Simon!

[Dmitry]

 Finally got a nRF52811, IQ sampling is awesome ) Still have no transmitter with antenna switch, so now playing with TX pattern.

My DFE configuration:

NRF_RADIO->DFEMODE = 2;
NRF_RADIO->DFECTRL1 = (16 << 0) | (0 << 7) | (2 << 8) | (6 << 12) | (0 << 15) | (6 << 16) | (0 << 24);
NRF_RADIO->DFECTRL2 = (0 << 0) | (0 << 16);

Here what I got with pattern F0 F0 FF F0 FF FF FF FF 00 FF ...

And again questions:

1) Range of I/Q values is about -150..+150, isn't it too low for 12-bit ADC? I use 10056 devkit as transmitter at 2 cm distance, TX power is 0 dbm, ADC backoff gain is 0.

2) I see there is 1-usec skip after 64 samples of reference period (8 usec). After that, a wave is continuous, so receiver does sampling even in switching slots? ok, it's very good for our case, just would like to be sure that it won't change in the next chip revision )

3) Still can't get TSAMPLEOFFSET. I figured out that changing it affects start of reference period (not start of sampling after it), and 0 corresponds something about 16 usec after ADDRESS event. Maybe it worth to update a documentation to make this item more clear?

Thanks!

[Simonr]

Hi

1. The range of I/Q samples depends on the AGC during the packet. It could be that the signal is so strong in this case that the gain is lowered a lot. Difficult to say from the description. One could separate the two devices and see if the I/Q sample range increases.
2. The settings are very different to what we typically use for the BLE specification and I got confused about the switching slot/state. For the BLE spec typically SWITCHSPACING = SAMPLESPACING and then there is no sampling in switching slots. In general there is sampling in the Reference period and in the "Switching period" (i.e. switching and sample slots). We should probably have called it "Switching state" to align with the DFESTATUS register description. The 1us skip is expected, as it skips like that to ensure the switching period starts at the same place even with different sample settings.
3. The documentation is a work in progress, and we will take your notes into account! For now, keep this value at 0 or 1, as these are the recommended values. Thanks for your input.

Best regards,

Simon

[Armin_B]

Hi Dmitry

how did get a nrf52811 board to test? and which library you used to configure the DFE and I/Q sampling?

Thanks