SFR02

[DanPhelan]

I connected the SFR02 sensor to a Synapse-wireless module with an I2C interface. The SFR02 accepts an i2c write command and takes readings, but reading the SFR02 module always returns a decimal 255 value. It seems like a timing issue? What is a likely cause of always reading 255? (The unit works fine using a serial port to PC.)

Thanks,
Dan

[chris]

Hi Dan,

Are you waiting 70ms after sending the transmit command before reading the result? The SRF02 will return 255 if there has not been sufficient time between the transmit and get range commands.

[asic_designer]

I though the amout of time you were susposed to was wait was 70us (microseconds) not ms(miliseconds) is that correct? The reason why I ask is I am currently having the exact same problem with this sensor, so I too would like clarification on this issue.

[chris]

The delay should be 70ms, the speed of sound is typically 343m/s in air so therefore 343*0.07 = 24.01m which is the sounds travel during that time. For ranging it also requires the time for the echo to travel back, therefore 24/2 = 12m of range. After this the echos will be to faint to trigger the SRF02 and it can complete its listening cycle. The SRF02 will actually finish its listening cycle nearer the 60ms mark (10.5m of range), but 70ms is a better value to use to ensure its done.