RF Design

1. TITLE : Implementing Wireless Remote Control Using Radio Frequency Module
NAME : Thian Nyuk How
Matrix Number : HK2006-3092
DATE : 24 MARCH 2009
Abstract:
This application note is about implementing wireless remote control using a Radio Frequency
(RF) module interface with microcontroller. Asynchronous mode in Universal Synchronous and
Asynchronous Receiver Transmitter (USART) will be introduced in interfacing the RF module with a
microcontroller.
1.0 Introduction:
Radio frequency (RF) usually refers to oscillations in electrical circuits. It is normally use in
wireless communication system to transmit data. By modulating data signals onto the RF carrier signal,
data can be transfer from one point to the other point easily. In this application note, a pair of RF module
with a frequency of 433 MHz is used.
To interface RF module with microcontroller, Universal Synchronous and Asynchronous
Receiver and Transmitter (USART) or also known as Serial Communications Interface is used. USART is
used for transmit and receive serial data. The operation of USART can be divided into two types which is
synchronous and asynchronous. Synchronous mode uses a clock and data line. Asynchronous mode does
not use clock accompanying the data.. However in this application note, only asynchronous mode will be
use in interfacing the RF module with the microcontroller.
2.0 Objective:
Interface a RF module with microcontroller to build a wireless remote control.

2. 3.0 Hardware:
RF Transmitter Module (433MHz)
Specifications RF Transmitter Module
Operating Voltage 3V to 12 V
Operating Current Max: 400mA for 12 V supply
Min: 9mA for 3V supply
Frequency 433MHz
Transfer Rate 10Kbps
Antenna Length 18cm
Table 1.1 Specification of RF Transmitter Module
There are 3 pins to connect in the RF Transmitter module. The DATA pin is connected to the TX
pin of the Microcontroller. VCC pin connect to the supply voltage and GND connect to ground. The
antenna can extend with any wire but for better result, a 50 Ohm coaxial cable is used. The length of the
wire as antenna is about 18cm long.
RF Receiver Module (433MHz)

3. Specifications RF Receiver Module
Operating Voltage 5.0V ± 0.5V
Operating Current ≤5.5mA for 5.0V supply
Frequency 433MHz
Transfer Rate 10Kbps
Antenna Length 18cm
Table 1.2 Specifications of RF Receiver Module
There are 5 pin to connect for the RF receiver module. The DATA pin of the RF Receiver
Module is connecting to the RX pin of microcontroller. The VCC pin connects to the 5v supply and the
GND pin to ground. The ANT is the antenna of the receiver and can be extend with any wire. The length
of the wire is 18cm for better result.
4.0Interface RF Transmitter and Receiver with Microcontroller:
4.1 Register and Flag bit Description which will be used :
Register Name Description
TXSTA Transmit Status and Control
RCSTA Receive Status and Control
TXREG Write Transmit Data Register
RCREG Write Receive Data Register
SPBRG Setting Baud Rate
PIR1 Peripheral Interrupt Flag Register
PIE1 Peripheral Interrupt Enable Registers
Table 1.3 Register and Description which will be used
Flag Bit Name Description
TXIF Located in PIR1 (bit 4) which is use to check
whether TXREG is Full or Empty
RCIF Located in PIR1 (bit 5) which is use to check
whether RCREG is Full or Empty
OERR To test over run error for the RCREG Register
TXEN Transmit Enable of Disable bit
Table 1.4 Description of flag bit that will be used

4. 4.2Interfacing Transmitter with Microcontroller PIC16F628A:
Figure 1.1 Remote Control (Transmitter circuit)
In the schematic of the remote control show in Figure 1.1, the microcontroller used is
PIC16F628A. To connect a RF transmitter module with PIC16F628A, the data pin of the transmitter need
to connect to the TX pin of the PIC which is pin 8 for PIC16F628A. TX pin is for the use of serial
communication interface which will transfer 8 bit data to the transmitter to be transmit. To enable the TX
port as a serial port, SPEN which is bit 7 in RCSTA have to be set. Bit two in TRISB of PIC16f628A
have two be clear in order to make the TX pin as an output pin. TXSTA is the transmit control register for
the PIC. This register has to be initializing correctly in order to make the transmitter work. By referring to
the data sheet, the source code in this application note, the TXSTA is initialized as B'00100000' which
mean that it transmit 8bit data in asynchronous low speed mode. Figure 1.2 shows the representation of
each bit of the TXSTA register. Next is to set the baud rate of the transmitter.

5. Figure 1.2 Representation of Each Bit in TXSTA Register
Baud rate refers to the speed at which the serial data is transferred, in bits per second. In Asynchronous
mode, the baud rate generator sets the baud rate using the value in the SPBRG register. The BRGH bit in
TXSTA selects between high and low speed options for greater flexibility in setting the baud rate. From
the initialization of TXSTA shown above, the BRGH is clear which mean that the baud rate is in low
speed and the SPBRG register in this application note’s source code is set to 255. The Baud rate for both
transmitter and receiver must be the same in order for the data transmitted to receive in the receiver.
When a digital data is being transmitted, it is transmit from the less significant bit to the most significant
bit. This means that the transmitter transmits digital data bit by bit to the receiver. Figure 1.3 show how
the signal is transfer in asynchronous mode.
Figure 1.3 Signal Transfer in Asynchronous mode

6. From Figure 1.3 the signal is high when no transmission (or reception) is in progress and goes
low when the transmission starts. The receiving device uses this low-going transition to determine the
timing for the bits that follow. The signal stays low for the duration of the START bit, and is followed by
the data bits, Least Significant bit first. The USART can transmit and receive either eight or nine data bits.
The STOP bit follows the last data bit and is always high. The transmission therefore ends with the pin
high. After the STOP bit has completed, the START bit of the next transmission can occur as shown by
the dotted lines.
During transmitting data, the heart of the transmitter is the Transmit Shift Register (TSR). This
register obtain the data from the transmit buffer, TXREG. Hence, to transmit a data to the receiver, first is
to move the desire transmit data to the TXREG then it will load to TSR to be transmitted. To check
whether the data in TXREG had been move to TSR, the flag bit TXIF which located in the PIR1 is
checked. If TXREG is empty (means the data already load to TSR) the flag bit TXIF will be set. Hence
new data can be load to TXREG to be transmitted next. The Bit TXEN in TXSTA show in Figure 1.2 is
always set so that all the data in TSR will be transmit. Appendix 8.1 shows the sample code of transmit a
data.
Check TXIF False
True
Move Digit 9
to TXREG to be
transmitted
Figure 1.4 Block Diagram Showing the transmitter’s program runs

7. 4.3 Interfacing RF Receiver Module with microcontroller:
Figure 1.5 A Testing Receiver Circuit (Receiver Circuit)
In the schematic of the receiver circuit show in Figure 1.5, the microcontroller used is
PIC16F877A. To connect the RF Receiver Module to PIC16F877A, the data pin of the receiver is
connected to the RX pin which is pin 26 for PIC16F877A. RX pin is for the use of serial communication
interface which will receive the 8 bit data from the transmitter for further process. Before the receiver able
to receive any data from the transmitter and provide to the PIC for further process, initialization for the
PIC is needed.
RCSTA is the receive control register for the PIC. This register has to be initializing correctly in
order to make the receiver work. By referring to the data sheet, for the source code of this application note,
RCSTA is initialized as B'10010000' which mean that it continuously receive 8 bit data, asynchronous
mode. The SPEN Flag bit in RCSTA have to be set to enable the serial port. The baud rate for the receiver
has to be the same with the transmitter. Figure 1.6 shows the representation of each bit of the RCSTA
register.

8. Figure 1.6 Representation of Each Bit in RCSTA Register.
When receiving data from the transmitter, the data is first stall in Receive Shift Register (RSR).
After that the received data is transferred to the RCREG register when it is empty. Once the transferring
process from the RSR to RCREG is complete, the flag bit RCIF will be set. The RCREG is a double-
buffered register which mean that it can store two byte of data. When the 2nd data come in but the 1st data
have not been read yet, the data will store in the second slot of the RCREG. When the 1st data is read, the
2nd data will move to the 1st slot and new data can be move into RCREG. However, when the RCREG is
full and the 3rd data is store in the RSR, the flag bit OERR will be set and the data in RSR will lost. In
addition, all the receive process will be stop. Hence it is a must to clear the flag bit OERR in order to
retrieve the receiving process. Flag bit OERR can be clear by first clear the CREN and then set it again.
Appendix 8.2 shows the sample code to receive date from receiver.

9. The test receiver circuit show in schematic is connected with two LED which is used to test
whether the receiver receive data from the transmitter. If it receive the correct data from the transmitter,
will either on or off based on the desire of user to program it.
False
Move data from
Check RCIF TRUE Identify the data receive
RCREG to a file
True
False
Activate
Dog Collar
Figure 1.7 Block diagram showing the receiver’s program runs
5.0 Result:
A brief experiment is conducted to test the range of the RF remote control range.
Condition Outcome
Without Antenna Receive Range less than 5 meters
With antenna in closed surrounding Receive range about 25 meters
With Antenna in open surrounding Receive range about 35 meters
Table 1.5 experimental result of the range of the RF remote control in different condition
Another experiment is conducted to test the interference upon the RF module with RF module of different
frequency. It is proved that the RF module will not be interfering by the RF module of different frequency.
However when two same frequency of transmitter is transmitting data, the receiver will receive error
signal which is not the exact signal from both transmitter.
6.0 Conclusion:
The RF module perform a satisfy range as a wireless remote control and able to transfer signal through
obstacles. Interfacing the RF module with microcontroller through USART shows a stable performance
from both the transmitter and receiver.

10. 7.0 References:
1. Microcontroller Programming, The Microchip PIC, Juliao Sanchez, Maria P.Canton (e-book)
2. Data Sheet Microchip PIC16F877A
3. Data Sheet Microchip PIC16F628A
8.0 Appendix:
8.1 Sample Assembly code of transmit a digit 9 continuously
Transmit ; to transmit a digit 9
BANKSEL PIR1
BTFSS PIR1, TXIF ;check flag bit TXIF
GOTO Transmit
MOVLW D'9' ; move digit 9 to TXREG
BANKSEL TXREG
MOVWF TXREG
GOTO Transmit
8.2 Sample code of Reciever Receiving Data 9 from Transmitter
Receive MOVLW D'9'
MOVWF H1 ; move digit 9 to file H1
BANKSEL PIR1
BTFSS PIR1,RCIF ; check if RCIF is set
GOTO $-1
BANKSEL RCREG ; receive complete
MOVF RCREG,W
MOVWF D1 ; move receive data to D1
MOVF D1,W
SUBWF H1,0 ; check receive data
BTFSC STATUS,Z
GOTO Receive ; wrong data re-loop
GOTO On_LED