Nuts and Volts - April 2017

A PW1-928 transmit sequence is initiated when the PIC’s UART is found to contain a received byte. The PIC enters transmit mode and the SX1272 is put into the “prepare to transmit” mode. Received data that is driven through the PIC’s UART is loaded into a different 64-byte FIFO for the 10 mS it takes the SX1272 to enter a stable ready state. When the SX1272 is deemed ready, up to 15 bytes of data are transferred from the transmit FIFO, formatted, and written to the SX1272.

Following the transfer of data from the transmit FIFO to the SX1272, the transmit command is issued and the SX1272 responds by transmitting a packet. The transmission process takes approximately 15 mS. While the transmit process is in progress, the PIC continues to read its UART and transfer any new data to the transmit FIFO. Data in the receive FIFO will be fed out through the UART during transmit, but new incoming packets cannot be processed until the transmit process has completed.

At the end of a transmission, if there is still any data in the transmit FIFO, another read FIFO/format/transfer/start transmit sequence is triggered. If the transmit FIFO is empty, the PW1-928 reverts to the receive mode.

Each PW1-928 is capable of being addressed using a four-bit (1 of 16) group address and a four-bit channel-group identifier. The group address and channel-group identifier addressing elements are combined into a single programmable ID byte.

Each PW1-928 also contains a unique 32-bit serial number which is factory programmed into the PW1-928’s internal EEPROM area. The PW1-928 serial number can be accessed by the host microcontroller via its RS-232 serial interface. This access to the 32-bit serial number allows the PW1-928 factory serial number to be used by the host application as a unique identifier.

The PW1-928 RF Data Transceiver Module Dev Kit

The PW1-928 module attaches easily to any microcontroller that is equipped with an embedded UART. However, if scratch building a PW1-928 host is not in the design cycle, bringing up a PW1-928 with its development kit is the easiest alternative option. The official Lemos International PW1-928 dev kit is shown in Photo 3.

The PW1-928’s component details are graphically illustrated in Schematic 1. Basically, the PW1-928 dev kit main board provides a place to mount a PW1-928

transceiver module plus its antenna, a regulated 3. 3 volt power supply, jumper access to all of the PW1-928’s indication signals, and a 3. 3 volt compatible serial interface. Power for the PW1-928 dev kit can be sourced by the kit’s onboard nine volt battery receptacle or via header pins on the dev kit main board. Note that the dev kit’s UART interface is composed of discrete transistors. There are also LEDs attached to the PW1-928 indication I/O pins.

Turning the PW1-928 Knobs

Figure 3 augments the information provided by Schematic 1. If we wanted to connect our PW1-928 dev kit directly to a host PC’s native RS-232 port (good luck finding one), we would use the PW1-928 RS-232 signals and ground associated with the dev kit’s JP1 interface. In this case, we are going to interface our PW1-928 to a terminal emulator running on a PC. Since my brand new Dell does not present nine discrete serial port pins in the