TXADDRESS, RXADDRESSES registers: what are these and how we can use them?

[Omid]

Based on the NRF Radio manual, the over the air address of the packets, is configured using "BASEn" and "PREFIXn" registers.

However, there are also two other registers called "TXADDRESS" and "RXADDRESSES".

 What is the application of these registers and how we can use them?

My current reading of the specification tells me that these are used to facilitate the filtering of the unwanted packets in the hardware level but I am not sure.

The BLE standard does not seem to have any instructions for this type of addresses (which is called logical addresses sometimes in the NRF Radio specification). Is this something that NRF has designed for extra flexibility and fast filtering and connection?

As far as APIs are concerned, I have found the following:

NRF_RADIO->RXADDRESSES = (uint32_t)(rxaddresses);

NRF_RADIO->TXADDRESS = ((uint32_t)txaddress) << RADIO_TXADDRESS_TXADDRESS_Pos;

 

Could you tell me what does the left shift operation in the second code do? (why we do that)

Thanks

[Omid]

For a better reference, I am literally copying the document:

---

The on-air addresses are defined in the BASE0/BASE1 and PREFIX0/PREFIX1 registers. It is only when writing these registers that the user must relate to the actual on-air addresses. For other radio address registers, such as the TXADDRESS, RXADDRESSES, and RXMATCH registers, logical radio addresses ranging from 0 to 7 are being used. The relationship between the on-air radio addresses and the logical addresses is described in the following table.

Logical address	Base address	Prefix byte
0	BASE0	PREFIX0.AP0
1	BASE1	PREFIX0.AP1
2	BASE1	PREFIX0.AP2
3	BASE1	PREFIX0.AP3
4	BASE1	PREFIX1.AP4
5	BASE1	PREFIX1.AP5
6	BASE1	PREFIX1.AP6
7	BASE1	PREFIX1.AP7

Table 1. Definition of logical addresses

I cannot simply find the answer of my questions from reading this paragraph.

Could you please translate this paragraph in plain language?

I honestly have no idea what "It is only when writing these registers that the user must relate to the actual on-air addresses" is meant to convey to the reader.

Håkon Alseth said:

The usage logical vs. on-air address are not mutually exclusive, they are linked to the addressing itself.

So, it means I can use one with or without the other on my wish????

Let's say I set up the BASE0 and PREFIX0 to create the air address of a packet to be transmitted.
However, I do not make any settings for TXADDRESS.
Will the transmission be done after TASKS_TXEN and TASKS_START? or it won't work?

Let's say I want to receive packets. 
However, I do not make any setting for RXADDRESSES.
Will the reception be done after TASKS_RXEN and TASKS_START? or it won't work?

[Håkon Alseth]

Hi,

 

Omid said:

Let's say I set up the BASE0 and PREFIX0 to create the air address of a packet to be transmitted.
However, I do not make any settings for TXADDRESS.
Will the transmission be done after TASKS_TXEN and TASKS_START? or it won't work?

This will leave the TXADDRESS with value '0' (ie. the reset value of the register), which will point to logical address #0, which again is BASE0 combined with PREFIX0.AP0 on-air address.

 

Omid said:

Let's say I want to receive packets. 
However, I do not make any setting for RXADDRESSES.
Will the reception be done after TASKS_RXEN and TASKS_START? or it won't work?

As compared to the TXADDRESSES register, which uses the value directly, the RXADDRESSES register is defined as bitfields.

This is because you can be in RX and receive payloads from RXADDRESSES that you have enabled.

 

If RXADDRESSES == 0, then you will not receive any payloads, as there's no addresses you're listening to.

Kind regards,

Håkon

[Omid]

Thank you Hokon, this makes it a lot more clear.

Just one more follow up question on:

Håkon Alseth said:

This is because you can be in RX and receive payloads from RXADDRESSES that you have enabled.

Does this mean, in the RX mode, we can listen to the air addresses that we provide?

If so, does it also mean we will only be able to listen to those addresses?

If so, I am wondering if it means that we can listen to several air addresses at the same time? depending how many base and prefix addresses we provided?

Thank you very much.

[Håkon Alseth]

Hi,

 

Omid said:

Does this mean, in the RX mode, we can listen to the air addresses that we provide?

Yes, that is correct.

 

Omid said:

If so, does it also mean we will only be able to listen to those addresses?

If so, I am wondering if it means that we can listen to several air addresses at the same time? depending how many base and prefix addresses we provided?

Yes, you can listen to up to 8 addresses in total.

One can ofcourse extend this to use parts of the user-payload as an "extended addressing" based on the BASE+PREFIX that is already setup in the NRF_RADIO peripheral, but this will require specific handling in firmware.

 

Kind regards,

Håkon

[Omid]

Thank Hokan,

Just a follow up question.

What I have realized, by trial and error, is that no matter what is used for base and prefix addresses, the radio always gives back the Address, Payload and End signals and populates the packet pointer, is this something expected? Because based on our discussion here, I am not suppose to get any populated packets if the addresses are not matching.

Well, at first I thought that might be just a coincidental match, but I tried several random and weird addresses, but it seems that the radio populates the packet pointer regardless. I believe it is very unlikely that I would always get such coincidental matches.

Also, I should mention that I have tried a very ridiculously large prefix and in that case the radio did not populate the pointer and the address, payload, and end signals are not received. However, I am not very sure about the fact that having such a long prefix is right thing to do.