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April 1, 2009

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VirtualWire

Mike McCauley

Documentation for the VirtualWire 1.3

communications library for Arduino.

1.0 Introduction

Arduino is a low cost microcontroller with Open Source hardware, see http://www.ardu-

ino.cc. VirtualWire is a communications library for Arduino that allows multiple Ardu-

ino’s to communicate using low-cost RF transmitters and receivers.

The document describes the VirtualWire library and how to install and use it.

2.0 Overview

VirtualWire is an Arduino library that provides features to send short messages, without

addressing, retransmit or acknowledgment, a bit like UDP over wireless, using ASK

(amplitude shift keying). Supports a number of inexpensive radio transmitters and

receivers. All that is required is transmit data, receive data and (for transmitters, option-

ally) a PTT transmitter enable.

It is intended to be compatible with the RF Monolithics (www.rfm.com) Virtual Wire

protocol, but this has not been tested.

Does not use the Arduino UART. Messages are sent with a training preamble, message

length and checksum. Messages are sent with 4-to-6 bit encoding for good DC balance,

and a CRC checksum for message integrity.

Why not just use the Arduino UART connected directly to the transmitter/receiver? As

discussed in the RFM documentation, ASK receivers require a burst of training pulses

to synchronize the transmitter and receiver, and also requires good balance between 0s

and 1s in the message stream in order to maintain the DC balance of the message.

Summary of content (13 pages)

PAGE 1
April 1, 2009 VirtualWire Copyright (C) 2008-2009 Mike McCauley Documentation for the VirtualWire 1.3 communications library for Arduino. 1.0 Introduction Arduino is a low cost microcontroller with Open Source hardware, see http://www.arduino.cc. VirtualWire is a communications library for Arduino that allows multiple Arduino’s to communicate using low-cost RF transmitters and receivers. The document describes the VirtualWire library and how to install and use it. 2.
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Overview UARTs do not provide these. They work a bit with ASK wireless, but not as well as this code. 2.1 Supported hardware. A range of communications hardware is supported. The ones listed blow are available in common retail outlets in Australian and other countries for under $10 per unit. Many other modules may also work with this software. Runs on ATmega8/168 (Arduino Diecimila etc) and ATmega328 and possibly others. 2.2 Receivers • RX-B1 (433.92MHz) (also known as ST-RX04-ASK) FIGURE 1.
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Overview FIGURE 2. TX-C1 Details at http://www.tato.ind.br/files/TX-C1.pdf 2.4 Transceivers: • DR3100 (433.
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Downloading and installation FIGURE 3. DR3100 Details at http://www.rfmonolithics.com/products/data/dr3100.pdf 3.0 Downloading and installation The latest version of this document is available from http://www.open.com.au/mikem/arduino/VirtualWire.pdf Download the VirtualWire distribution from http://www.open.com.au/mikem/arduino/VirtualWire-1.3.zip To install, unzip the library to a sub-directory of the hardware/libraries subdirectory of your Arduino application directory.
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Function calls 4.1 vw_set_tx_pin extern void vw_set_tx_pin(uint8_t pin); Set the digital IO pin to use for transmit data. Defaults to 12. 4.2 vw_set_rx_pin extern void vw_set_rx_pin(uint8_t pin); Set the digital IO pin to use for receive data. Defaults to 11. 4.3 vw_set_ptt_pin extern void vw_set_ptt_pin(uint8_t pin); Set the digital IO pin to use to enable the transmitter (press to talk). Defaults to 10. Not all transmitters require PTT. The DR3100 does, but the TX-B1 does not. 4.
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Sample code Block and wait until the transmitter is idle 4.9 vw_wait_rx xtern void vw_wait_rx(); Block and wait until a message is available from the receiver. 4.10 vw_wait_rx_max extern uint8_t vw_wait_rx_max(unsigned long milliseconds); Wait at most milliseconds ms for a message to be received. Return true if a message is available. 4.11 vw_send extern uint8_t vw_send(uint8_t* buf, uint8_t len); Send a message with the given length.
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Sample code void loop() { const char *msg = "hello"; vw_send((uint8_t *)msg, strlen(msg)); delay(400); } 5.2 receiver A simplex (one-way) receiver. Waits for a message and dumps it contents. Test this with transmitter above. #include void setup() { Serial.begin(9600); Serial.
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Sample code const char *msg = "hello"; uint8_t buf[VW_MAX_MESSAGE_LEN]; uint8_t buflen = VW_MAX_MESSAGE_LEN; vw_send((uint8_t *)msg, strlen(msg)); vw_wait_tx(); // Wait until the whole message is gone Serial.println("Sent"); // Wait at most 400ms for a reply if (vw_wait_rx_max(400)) { if (vw_get_message(buf, &buflen)) // Non-blocking { int i; // Message with a good checksum received, dump it. Serial.print("Got reply: "); for (i = 0; i < buflen; i++) { Serial.print(buf[i], HEX); Serial.print(" "); } Serial.
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Implementation Details for (i = 0; i < buflen; i++) { Serial.print(buf[i], HEX); Serial.print(" "); } Serial.println(""); // Send a reply vw_send((uint8_t *)msg, strlen(msg)); } } 6.
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Connections Testing with TX-C1, RX-B1, 5 byte message, 17cm antenna, no ground plane, 1m above ground, free space At 10000 bps the transmitter does not operate correctly (ISR running too frequently at 80000/sec?) At 9000 bps, asymmetries in the receiver prevent reliable communications at any distance At 7000bps, Range about 90m At 5000bps, Range about 100m At 2000bps, Range over 150m At 1000bps, Range over 150m As suggested by RFM documentation, near the limits of range, reception is strongly influenced by
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Connections 8.1 RX-B1 receiver FIGURE 4. Wiring for RX-B1 receiver Arduino 11 Data 5V Vcc Gnd Gnd RX-B1 Ant 8.2 TX-C1 transmitter FIGURE 5.
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Connections 8.3 DR3100 transceiver FIGURE 6. Wiring for DR3100 Arduino 4 DR3100 Rx Data 5 Tx In 10 11 Ctr1 3V3 9 Vcc 12 Ctr0 11 12 12k 1 Gnd 10 FIGURE 7.
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Copyright and License Connections shown for no AGC, 1mW power out, 2400bps. Note the 12k resistor in series with Tx In to control the power output If you want to use higher data rates, need a resistor from pin 8 of the DR3100 to ground (see RFM documentation for more details). If you want to use AGC, need a capacitor from pin 1 of the DR3100 to ground (see RFM documentation for more details). The DR3100 module is supplied without any connection pins, only surface mount style pads.