Replacing RS485 link with NRF24L01+

[o.hojvat]

As Pevious ticket: Using NRF24L01+ with pure assembler programing becomes very long,  I open a new one as Torbjørn suggested

This is a description about what I have doing in my proyect and at the end there are some questions.

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My application has a Central unit connected to several Handheld controllers  with RS485 cable.
Bit rate is 62.500 , 9bits

The central unit is sending in secuence a Callbyte for each Handheld address (from1 to 31) and wait about 100 us for a reply, before it will send a Callbyte to the following address
In this cable system a Handheld only changes to Tx mode after a Callbyte with his address is received.

The Central unit has connected two rails for Model Electric trains. (Ferromedelism)

I will add an interfase  with a uP and a NRF24L04p that will handle the RF link and connected to the Central by 485 cable
No modifications at the Central unit hadware or software willbemade.

The the handheld will be re-designated by adding one NRF24L01+, removing the 485 part and modifying the program as needed.

Using the usual star configuration , the intefase will use PRX mode and may be up to 6 handheldwill using the NRF part in PTX mode.

All the 485 packets are 9 bits but the MSB is only for the information about if the byte is an Address (= 1) or a DATA  (= 0)

Only the Central may send Address and Data bytes.   Handheld units only send DATA bytes

In my aproach the interfase unit decodes only the Callbytes and the NRF will need to handle only 8 bits.

There are 3 types of Callbytes:

1) Callbytes with no more bytes in the packet, that allows the matching address Handheld to

send a packet to the central

2) Callbytes with 1 or more DATA with information requested before by one specific handheld.

The regarding address will be in the Callbyte

3) Broadcast Callbytes: The packet will have information that must be sent to all the Handhelds units.

The program for the interfase unit has 6 RX buffers and 6 TX buffers for the USART and 485 , one pair for each PIPE

Data send from a Handheld by NRF is received by interfase and stored in the TX buffer , for case 1)

Data received by USART (from RS485) is stored in Rx buffer and loaded in NRF TXFIFO  for sending it as an ACK PAYLOAD

I will use some time slots system (not defined yet) for each PTX for avoid collitions and to get back  information when available

------------------------- Some quetions here ---------------------

At the moment the matter  1) is working OK   

Each information sent from Handheld is received at interfase side and sent by RS485 when type 1) Callbyte arrives.

For cases 2) and 3)  ,  information from Handheld is received but ACK PAYLOAD is sent some times yes  other times no

uP reset do not helps .  If the NRF supply is disconected and reconected, ACK PAYLOAD begins to work

It may be  working in all the PTX packets or alternately one yes one not.

I have a low cost , 8 channels Logic Analizer and I will investigate this problem by reading STATUS and FIFO_STATUS

I guess the problem is at PRX side, not sending ACK, but I am no sure.

Please send any suggestion about where it may be the problem.

Also how to diferenciate  which side is not working well.

Best Regards, Osvaldo Hojvat

[o.hojvat]

There is a time of about 130us for PRX changes from Rx to Tx

Meanwhile:

For SPI it is posible to do an upload for the ack payload (6 byte packet) in about  20 us, inmediately IRQ of PRX goes low.

But may it works ?

Regards, Osvaldo

[ovrebekk]

Hi Osvaldo

You should be able to buffer regular payloads (PTX) or ACK payloads (PRX) independently of what the radio is currently doing. As an example, it is fine to upload multiple payloads while the radio is busy ramping up. 

o.hojvat said:

For SPI it is posible to do an upload for the ack payload (6 byte packet) in about  20 us, inmediately IRQ of PRX goes low.

You mean the IRQ goes low immediately after uploading an ACK payload? 
This sounds odd. 

On the PRX side you should not expect an interrupt unless you receive a packet from the PTX, uploading an ACK payload on the PRX side will have no influence on when the PTX sends you data. 

Best regards
Torbjørn

[o.hojvat]

I will try to explain myself better:

     I know that the ack payload must be loaded before receiving the packet in the PRX, so that it goes transmitted together with the next ACK.

     In the data sheet I can't find information about time margins regarding the IRQ go to low level and the upload timming

    What happens if the upload is done in the first 20 us after the IRQ goes low at PRX side ?

     The RF part takes 300 us to go to TX (PLL lock) and may be at the beginning of this time the payload can be loaded in the TX FIFO, to be transmitted next .

  I can do some tests, but perhaps this matter has already been analyzed

Best Regards, Osvaldo Hojvat

[o.hojvat]

Hi Torbjørn

I have many parts of my project working as expected.

There is a disappointing problem:  the system works but it depends on the order of PRX or PTX started first.

The system may be locked because tx FIFO or Rx FIFO full or Max retry number.

 Since I have a Tx from PTx each 128ms , I can do some FLUSH for FIFOS and clear STATUS flags on PRX and/or PTX sides if there are no packets for example in 200ms.

 I am asking if there are some recommendations that are usual for this matter.

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By the way I have one idea to share

        About  SPI speed,  working with PIC18F26Q10  I have used 16 MHZ sending data to a flash memory and 5MHz when receiving data.    This meets time specifications for both parts and works fine.

It is usefull in my aplication because I send 4bytes for command and address and get 4 data bytes in each access for reading. 

For NRF I used 4MHz by the moment..

Best Regards

 

[ovrebekk]

Hi Osvaldo

o.hojvat said:

    What happens if the upload is done in the first 20 us after the IRQ goes low at PRX side ?

In order for the packet upload to be included in the ACK the entire packet upload procedure has to complete before the IRQ goes low as a result of an incoming packet. If the packet upload completes after the IRQ has gone low it is already too late, and the packet will only be returned on the next packet from the PTX. 

o.hojvat said:

The RF part takes 300 us to go to TX (PLL lock) and may be at the beginning of this time the payload can be loaded in the TX FIFO, to be transmitted next .

Unfortunately you can not upload a packet while the TX is settling and have it included immediately. If you check the state diagram in chapter 7.5.2 you will see that the FIFO status is checked before enabling the radio in TX mode. If there are no packets in the TX FIFO the radio will go back to RX mode immediately:

Also, please note that TX settling time is 130us, not 300us. 

o.hojvat said:

There is a disappointing problem:  the system works but it depends on the order of PRX or PTX started first.

The system may be locked because tx FIFO or Rx FIFO full or Max retry number.

If you get into problems when starting the PRX first you could put some code on the PRX side to avoid uploading any ACK payloads until at least one packet is received from the PTX. Then you know that you won't fill the TX FIFO on the PRX side until the PTX is ready to pick them up. 

o.hojvat said:

The system may be locked because tx FIFO or Rx FIFO full or Max retry number.

 Since I have a Tx from PTx each 128ms , I can do some FLUSH for FIFOS and clear STATUS flags on PRX and/or PTX sides if there are no packets for example in 200ms.

Yes, you can use a timeout on the PRX side to detect situations where the PTX is not sending any data for a long time, and flush the FIFO's accordingly. 

If nothing is received for a certain amount of time you might want to just flush the FIFO and immediately upload a new, updated ACK payload. As long as nothing is received simply repeat this procedure. If nothing is received for a very long time you might consider using a longer timeout value, until the PTX starts responding again. 

Best regards
Torbjørn