3.2 Implementing the Nrf24l01 Radio modules
3.2.2 Nrf24l01 Transmitter (TX Module
For proof of concept, dummy data was transmitted via this device. This hardware implementation was nrf24l01 module interfaced to a Nuceo board via SPI. This board would transmit this Dummy data.
Nucleo nrf2401l Transmitter Module
Setting up this module is relatively straightforward as numerous libraries are written for the nRf2401l module using the Nucleo Board. So all that is required is to import these libraries associated with this board and start using the functions of this board to achieve the needed functionality.
Nucleo Transmitter Software:
The software module in Appendix C simulates the dummy data transmitted via the nrf2401l wireless module. (Nordic, 2008)The device simulates the dummy data represented by reading an analogue pot value. The Nucleo board, together with the nRf2401L module, will be set up as a transmitter. Before any coding of the Nucleo, it is necessary to import the Nrf2401l Library to the IDE so that it can be used in the code:
Operating modes
If the device is in receiving mode:
If CE is high.Allows you to monitor the airwaves to receive packets
If CE goes low, it puts the device in Standby mode and no longer monitors the airwaves.
If Device in Transmitting mode:
CE is held low except when we want to transmit a packet
CSN is the enable pin for the SPI bus. This pin is active low.
SCK is the serial clock for the SPI bus
On the microcontroller, SPI configuration CPOL and CPHA must be configured.
SPI must be set up for 8bits
SPI of the microcontroller to be set as master and set up as a slave on nRf2401l
- 57 - To receive data on the nRF2401l, the following must happen:
CSN must be high, to begin with
The assert CSN low to alert nrf2401l that it will start receiving SPI data.
This pin will stay low throughout the transaction.
The hardware is straightforward to set up, and the pins are self-explanatory.
We will, however, mention and explain that this module is interfaced with the stm32f mother module via the SPI interface.
The SPI interface allows the device to read/write registers, transmit data and receive data
To send data to or receive data from the SPI port on the 24L01, the device had to be configured to do a few things.
i. SPI Instruction set for the nRF2401
The CSN pin on the 24L01 must be high to start with,
Assert the CSN pin low to alert the 24L01 that it is about to receive SPI data. Every time the device wants to send SPI commands, CSN Pin needs to go low.
Then transmit the command byte of the instruction you wish to send.
When receiving data bytes for this instruction, send one byte to the nRF24L01 for every byte that the device wishes to get out of the nRF24L01.
If sending the nRF24L01 data, simply send data bytes and generally don’t worry about what gets sent back to you.
(Nordic 2016)When receiving data from the nRF24L01, it makes absolutely no difference what is contained in the data bytes that have been sent after the command byte, just so long as the correct number of them has been sent.
ii. nRF2401l Data Pipes
When setting up the wireless module for communication, a few parameters have to be configured to get that from one device to the other. The key parameters:
Channel: which is the specific frequency channel that communication will take place on. Integers from 0 to 125(7 bits of the particular register) represent frequencies that are mapped
Reading pipe: the reading pipe is a unique 24, 32, or 40-bit address from which the module reads data(reading and writing pipes addresses are swapped between master and slaves when setting up a single channel wireless link)
Writing pipe: the writing pipe is a unique address to which the module writes data
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Power Amplifier (PA) level: the PA level sets the chip's power draw and transmission power. For this research (use with the Arduino), we will use the minimum power setting.
The writing and reading pipe addresses are swapped between the two radios communicating with each other, as the writing pipe for each radio is the reading pipe for the other. If this is not done, we will not have a successful connection.
Table 3.1 Instruction Set for nRf2401L
Scenario:
o The device wants to read the contents at a specific register at a known memory address 0x10.
o First, bring CSN low and then send the command byte ‘00010000’ to the 24L01. This instructs the 24L01 that the device wants to read register 0x10, which is the TX_ADDR register.
o From the table above from command “00010000.”
- 59 - o The first three bits 000 translates to read
o The last five bits in the 10000 in binary is 0x10 in hex.
o To sum up the instruction set bluntly, the first 3 bits are “what to do,” and the next 5 bits are “where to do.”
Then send five dummy data bytes, and the 24L01 will send back the contents of the TX_ADDR register.
Finally, bring the CSN pin back high. All totalled, the device will receive six bytes.
When any command byte is sent, the nRF24L01 always returns the STATUS register.
After that, we will have received the five bytes that are contained in the TX_ADDR register (Nordic Semiconductor Inc. 2009).
Table 3.2 Configuration Register of nRF2401L (Nordic, 2008)
- 60 - Understanding the datasheets and the technology itself is essential to writing the code/firmware to get these modules to operate. It also implements some form of debugging to visualize the successful setting of the radio interface before the modules can start sending and receiving data. See appendix for the code to set up the transmitter module.
iii. STM32f4 Receiver Software:
The libraries used to initialise and set up the Nucleo board are the same as those used to set up the STM32f4 Discovery boards because they have the same core processors. Similarly, for the stm32f4, there are also libraries already written for the nRf2401lL device. All that has to be done is download the library, include it in the project, and use the functions. The apparent difference between the Nucleo board and the STM32f4 discovery board is that the Nucleo board is Mbed enabled and the Discovery board is not, but fortunately, both boards are linked to STMCubemax so that the peripherals are accessible via Hal libraries associated with the boards. In other words, the Hardware abstraction layer libraries are available to implement code for initialising the peripherals of the devices.
Not forgetting to interchange the address pipes between the receive and transmit modules.
The libraries and functions associated with the NRf24l01 radio module for the Nucleo board and the STM 32f4 discovery board are interchangeable.
Rf Module is set data rate of 2Mbps and output Power of 0dbm.
iv. Setting up the Stmf4 discovery board as Receiver module.
As mentioned above, there are libraries written and are freely available for use with the STM32f4 microcontroller. The following pseudo-code is necessary to get the asset tracking device to operate as the receiver.
Import the necessary libraries into the project folder.
Include the necessary header files to be used with the radio module.
Set the respective Tx address and Rx Address in the receiver module.
Initialise the buffer in the code i.e. char Bio_ dataOut[32] and char Bio_dataIn[32]
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The datasheet uses an appropriate pin for SPI transfer and DMA mapping to choose available pins for the Nrf24l01 module.
Some radio modules have a pins pack function; this function can be “called” to set up the pins for the w module.
Set data rate and power level of the receiver in the module; it is necessary to set data rate to 2mbps and power level to -18dBm.
The while loop for the data module will look something like the code Appendix D.