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SRF08 Ultra sonic range finder

Technical Specification

Communication with the SRF08 ultrasonic rangefinder is via the I2C bus. This is

available on popular controllers such as the OOPic and Stamp BS2p, as well as a wide

variety of micro-controllers. To the programmer the SRF08 behaves in the same way as

the ubiquitous 24xx series eeprom's, except that the I2C address is different. The default

shipped address of the SRF08 is 0xE0. It can be changed by the user to any of 16

addresses E0, E2, E4, E6, E8, EA, EC, EE, F0, F2, F4, F6, F8, FA, FC or FE, therefore

up to 16 sonar's can be used. In addition to the above addresses, all sonar's on the I2C bus

will respond to address 0 - the General Broadcast address. This means that writing a

ranging command to I2C address 0 (0x00) will start all sonar's ranging at the same time.

This should be useful in ANN Mode (See below). The results must be read individually

from each sonar's real address. We have examples of using the SRF08 module with a

wide range of popular controllers.

Connections

The "Do Not Connect" pin should be left unconnected. It is actually the CPU MCLR line

and is used once only in our workshop to program the PIC16F872 on-board after

assembly, and has an internal pull-up resistor. The SCL and SDA lines should each have

a pull-up resistor to +5v somewhere on the I2C bus. You only need one pair of resistors,

not a pair for every module. They are normally located with the bus master rather than

the slaves. The SRF08 is always a slave - never a bus master. If you need them, I

recommend 1.8k resistors. Some modules such as the OOPic already have pull-up

resistors and you do not need to add any more.

Registers

The SRF08 appears as a set of 36 registers.

Location Read Write

Software

Revision

Command Register

Summary of content (7 pages)

PAGE 1
SRF08 Ultra sonic range finder Technical Specification Communication with the SRF08 ultrasonic rangefinder is via the I2C bus. This is available on popular controllers such as the OOPic and Stamp BS2p, as well as a wide variety of micro-controllers. To the programmer the SRF08 behaves in the same way as the ubiquitous 24xx series eeprom's, except that the I2C address is different. The default shipped address of the SRF08 is 0xE0.
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1 2 3 ~~~~ 34 35 Max Gain Register (default 31) 1st Echo High Range Register Byte (default 255) 1st Echo Low N/A Byte ~~~~ ~~~~ 17th Echo N/A High Byte 17th Echo N/A Low Byte Light Sensor Only locations 0, 1 and 2 can be written to. Location 0 is the command register and is used to start a ranging session. It cannot be read. Reading from location 0 returns the SRF08 software revision. By default, the ranging lasts for 65mS, but can be changed by writing to the range register at location 2.
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170 0xAA 2nd in sequence to change I2C address Ranging Mode To initiate a ranging, write one of the above commands to the command register and wait the required amount of time for completion and read as many results as you wish. The echo buffer is cleared at the start of each ranging. The first echo range is placed in locations 2,3. the second in 4,5, etc. If a location (high and low bytes) is 0, then there will be no further reading in the rest of the registers.
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actually capable of. It is possible to reduce the time the SRF08 listens for an echo, and hence the range, by writing to the range register at location 2. The range can be set in steps of about 43mm (0.043m or 1.68 inches) up to 11 metres. The range is ((Range Register x 43mm) + 43mm) so setting the Range Register to 0 (0x00) gives a maximum range of 43mm. Setting the Range Register to 1 (0x01) gives a maximum range of 86mm. More usefully, 24 (0x18) gives a range of 1 metre and 140 (0x8C) is 6 metres.
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10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 0x0A 0x0B 0x0C 0x0D 0x0E 0x0F 0x10 0x11 0x12 0x13 0x14 0x15 0x16 0x17 0x18 0x19 0x1A 0x1B 0x1C 0x1D 0x1E 0x1F Set Maximum Analogue Gain to 133 Set Maximum Analogue Gain to 139 Set Maximum Analogue Gain to 145 Set Maximum Analogue Gain to 152 Set Maximum Analogue Gain to 159 Set Maximum Analogue Gain to 168 Set Maximum Analogue Gain to 177 Set Maximum Analogue Gain to 187 Set Maximum Analogue Gain to 199 Set Maximum Analogue Gain to 212 Set
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LED The red LED is used to flash out a code for the I2C address on power-up (see below). It also gives a brief flash during the "ping" whilst ranging. Changing the I2C Bus Address To change the I2C address of the SRF08 you must have only one sonar on the bus. Write the 3 sequence commands in the correct order followed by the address. Example; to change the address of a sonar currently at 0xE0 (the default shipped address) to 0xF2, write the following to address 0xE0; (0xA0, 0xAA, 0xA5, 0xF2 ).
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Operation Current Duration Ranging command received - Power on 275mA 3uS +/- 10v generator Stabilization 25mA 600uS 8 cycles of 40kHz "ping" 40mA 200uS 65mS Ranging 11mA max Standby 3mA indefinite The above values are for guidance only, they are not tested on production units. Changing beam pattern and beam width You can't! This is a question which crops up regularly, however there is no easy way to reduce or change the beam width that I'm aware of.