January 2006, Issue 186

Electronic Scarecrow

CHECKING FOR REPLIES

After all the sensors have been read, the main routine checks if the latest outgoing packet has been acknowledged. Normally, the ACK packet should arrive while the sensors are being read. Each request packet (except for resent duplicates) has a unique sequence number with a matching reply having the same number. If these numbers don’t match, it means a reply has been lost. If this occurs too many times, the transmit power increases unless it’s already at its maximum. In this sort of case, you can only assume an unrecoverable problem, so the remote will sleep for 1 h before retrying.

When a valid reply is received, the routine checks if it’s a block of download data. If it is and its data length is nonzero, the data is written to the sound file download buffer in flash memory. A zero length download packet indicates the end of data and contains a checksum. If it’s a code update, the buffer needs to be copied to the executable code area, overwriting the existing code. The bootloader handles this and will reset the system if the update succeeds.

Finally, the code checks the packet’s flags. Two flags control output actions, activating either the auxiliary output or the sound playback function. The playback routine looks for a wav formatted file in the sound file flash memory buffer area, which must be in 8-bit 8-kHz PCM format. If the file is found, it turns on the external amplifier and plays the sound file. The file can be looped several times as specified by the packet flags to produce up to 5 s of sound. A third packet flag enables Download mode, which erases the download buffer and prepares to receive a new data file.