SRF01 ultrasonic range finder technical documentation overview the SRF01 is a single transducer ultrasonic rangefi nder in a very small footprint pcb. connection is t o a single pin serial interface. the serial interface i s a standard ttl level uart format of 1 start, 1 st op and no parity bits, and may be connected directly to many controllers, such as the basic stamps or the picaxe . baud rate at power-up is always 9600, there is a command to change this to 19200 or 38400 baud if preferred . up to 16 SRF01's may be connected together on a single bu s. like all our rangefinders, the SRF01 can measure in cm or inches. power requirements the SRF01 accepts any voltage between 3.3v and 12v, however the recommended maximum is 5.5v. internally, it operates at 3.3v and there is a low dropout regulator already on the SRF01 to provide t his. the serial i/o pin operates at 3.3v and also 5v toleran t, so you can connect it directly to 3.3v or 5v sig nals. operating current during ranging is 25ma, 11ma in s tandby (waiting for a command) and around 55ua in sleep mode (shutdown). single pin serial communication serial data is 1 start, 1 stop and no parity bits. serial data is a ttl level signal - it is not rs232 . do not connect the SRF01 to an rs232 port - you will destr oy the module! communication with the SRF01 is with both serial input and serial output on a single pin . the SRF01 will be listening at all times except w hen it is actually sending data, and will go back to listenin g as soon as its finished. to communicate with the SRF01, you simply need to send a "break", followed by two bytes, the address of the SRF01 (factory default is 1) and the command. a "break" is just a low level for 12 b it times or longer. 1.5ms or more will be fine. it is used to synchronize transfers on the 1-wire serial bus. the default shipped address can be changed by the user to any of 16 addresses 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 or 16, and therefore up to 16 sonar's c an be used. note - this range is different to the srf02 which has ad dresses 0-15. the reason is that for specific comma nds only all SRF01's regardless of their programmed add ress, will respond to address 0. this makes it easy to start all SRF01s ranging at the same time. baud rate the serial baud rate on the SRF01 always starts at 9600 on power up. it can be changed to 19200 or 384 00 if you wish. it will revert to 9600 at the next power up. to change the baud rate, just send a "break" fo llowed by the general call address of zero (not the module ad dress), then command 0x64 for 19200 or command 0x65 for 38400. you can only send the command to address zer o to change all SRF01's at the same time. connections the connections to the SRF01 are shown below. if yo u're using multiple SRF01's, you can connect them a ll up to the same serial port on your controller. just ma ke sure all the SRF01's are programmed to different addresses. there is a weak pull - up resistor (47k) to 3.3v on the module, to pull th e tx line up to a logic high
level when none of the SRF01's are transmitting. ou r 3 - way jst connectors are suitable for connecting the SRF01. commands to send a command to the SRF01, you need to send a "break" followed by two bytes. the first is the srf 01's address 1 to 16, (0x01 to 0x10) and then the actual command itself - see below. there are two commands to initiate a ranging (80 to 81) giving results in inc hes and centimeters. these two commands don't trans mit the result back to your controller. you should wait 70m s and then use command 94 to get the result of the ranging. another set of two commands (83 to 84) do the same, but also transmits the result of the ranging back to your controller as soon as it is available. together, th ese four commands (80,81,84 & 84) are called "real" because they perform a complete ranging. there is another s et of four commands (86, 87, 89 & 90) called "fake" . they are the same as the "real" commands except that the y do not send the 8-cycle ultrasonic burst out. the se are used where the burst has been transmitted by anothe r sonar. it is up to you to synchronize the command s to the two sonar's. there is a command (92) to transmit a burst without doing the ranging. command 93 is used to get the firmware revision of the SRF01. command 94 gets returns two bytes (high byte first) from the most recent ranging. put them together to make a 16-bit result. command 95 gets the status byte. bit 0 is the "lock " bit. it is low (0) after power up, after the "unl ock" command is executed and after the module exits slee p mode. it goes high after 6 rangings have been com pleted with at least 30cm of free space in front of the so nar. see advanced mode for more details. command 96 is used to place the SRF01 in a low powe r sleep mode. see sleep mode for more details. commands 98 & 99 are used to enable/disable advance d mode. command address 0 access action decimal hex 80 0x50 yes real ranging mode - result in inches 81 0x51 yes real ranging mode - result in centimeters 83 0x53 no real ranging mode - result in inches, automatically tx range back to controller as soon as ranging is complete. 84 0x54 no real ranging mode - result in centimeters, automatically tx range back to controller as soon as ranging is comp lete. 86 0x56 yes fake ranging mode - result in inches 87 0x57 yes fake ranging mode - result in centimeters
none of the commands which return data can use addr ess 0. that would mean SRF01's trying to transmit d ata at the same time, and is prevented. most commands which do not return data are allowed to use address 0. exceptions are address change commands. the advantage of allowing address 0 access is that by sending a ranging command such as 0x51 (81 decim al), all SRF01s will start ranging at the same time. aft er 70ms, you can then get the results from each srf 01 at its own address. led the led gives a brief flash during the "ping" whils t ranging. if the transducer has not locked, the le d is on with a brief flash off. if the transducer is locked, the led is off with a brief flash on. if you power up the SRF01 without sending any comma nds, it will flash its address out on the led. one long flash followed by a number of shorter flashes indic ating its address. the flashing is terminated immed iately on sending a command sequence to the SRF01. 89 0x59 no fake ranging mode - result in inches, automatically tx range back to controller as soon as ranging is complete. 90 0x5a no fake ranging mode - result in centimeters, automatically tx range back to controller as soon as ranging is comp lete. 92 0x5c yes transmit an 8 cycle 40khz burst - no ranging takes place 93 0x5d no get software version - sends a single byte back to the controller 94 0x5e no get range, returns two bytes (high byte first) from the most recent ranging. 95 0x5f no get status, returns a single byte. bit 0 indicates "transducer locked", bit 1 indicates "advanced mode" 96 0x60 yes sleep, shuts down the SRF01 so that it uses very lo w power (55ua). 97 0x61 yes unlock. causes the SRF01 to release and re - acquire its "transducer lock". used by our factory test routine s. 98 0x62 yes set advanced mode (factory default) - when locked, SRF01 will range down to zero. 99 0x63 yes clear advanced mode - SRF01 will range down to appr ox. 12cm/5in, 100 0x64 yes changes the baud rate to 19200 101 0x65 yes changes the baud rate to 38400 160 0xa0 no 1st in sequence to change i2c address 165 0xa5 no 3rd in sequence to change i2c address 170 0xaa no 2nd in sequence to change i2c address address long flash short flashes decimal hex 1 01 1 0 2 02 1 1 3 03 1 2 4 04 1 3 5 05 1 4 6 06 1 5 7 07 1 6 8 08 1 7 9 09 1 8 10 0a 1 9 11 0b 1 10 12 0c 1 11
changing the SRF01 address to change the address of the SRF01 you must have on ly one sonar connected. write the sequence of three commands in the correct order followed by the addre ss. example; to change the address of a sonar curre ntly at 1 (the default shipped address) to 5, write the fol lowing to address 1; (0xa0, 0xaa, 0xa5, 0x05 ). the se commands must be sent in the correct sequence to ch ange the address, additionally, no other command ma y be issued in the middle of the sequence. the sequence must be sent as four separate commands to the curre nt address of the sonar. i.e. "break" 0x01, 0xa0 then "break" 0x01, 0xaa, then "break" 0x01, 0xa5 and fin ally "break" 0x01, 0x05. when done, you should label the sonar with its new address. take care not to set m ore than one sonar to the same address, there will be a bus collision and very unpredictable results. the new address is stored in flash memory on the SRF01 and will remain, even if power is removed, until delibe rately changed. standard mode in standard mode the SRF01 measures from approx 18c m (7 inches) to 600cm (19ft). it requires no calibr ation or tuning to achieve this range. standard mode is r ecommended for commercial applications where the requirements of advanced mode cannot be met. to set standard mode send the clear advanced mode command, 99 decimal or 0x63. advanced mode this is the factory default mode. in this mode it i s possible for the SRF01 to measure range all the w ay down to zero. initially, the minimum range the SRF01 can de tect is around 18cm or 7 inches. for the SRF01 to b e able to measure all the way down to 0cm, it acquires a p recision crystal lock on the transducer's character istics. this happens after 5 rangings have been completed with 3 0cm or more of free space in front of the sensor. w hen this happens the "lock" bit is set (bit 0 in the st atus byte) and the SRF01 can measure from 0 to 600c m (0- 6meters). during operation the SRF01 maintains the transducer lock automatically, provided it gets an occasional (every few minutes) set of 5 consecutive rangings of great er than 30cm. the "lock" state of the SRF01 can be checked in by reading the "lock" bit, which is bit 0 in the status register. a high (or logic 1) indicates tra nsducer lock has been achieved and the SRF01 will work down to 0 cm. where the SRF01 is not able to maintain its loc k, it will release it (lock bit = zero) and re-acquire it as soon as possible. sleep mode when placed in sleep mode, all activity on the srf0 1's processor ceases. the module shuts down and pow er consumption is reduced to a minimum. this is just a combination of quiescent current in the regulator and leakage current of the processor. typically 55ua. t o wake the SRF01 up from sleep mode send 0xff. don' t send anything else - no "break", no address, becaus e the 0xff byte is not a command, it's really just a single transition on the rx line that wakes up all SRF01's on the serial bus. after waking up the SRF01, you should wait 2ms for the processor clocks to stabilise befo re sending commands. testing 13 0d 1 12 13 0e 1 13 15 0f 1 14 16 10 1 15
we have a small test program, written in c# which c ontrols the SRF01 via the acroname s22 module. a small schottky diode is used to prevent the SRF01 trying to drive the s22's output line (data from the pc). you will need to download and install the ftdi vcp drivers to use the s22. the complete project directory, with all source fil es, can be downloaded here . to use this program you should download and install microsoft visual c# 2008 . mounting the SRF01 a rubber grommet is supplied to mount the SRF01. to use this make a 20mm cutout in your panel and fit the grommet. the SRF01 will then snuggly push into the hole in the grommet. maximum panel thickness is 1.7mm. examples further examples of using the SRF01 can be found on our examples page.
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