The time taken for the player to load tracks depends on how far 'into' the disk they are. Track 001.mp3 in folder 01 may take less than a second to load. Track 001.mp3 in folder 25 may take 30 seconds (or more) to load. I had to build a variable delay for detecting the busy signal to take account of this. It is also possible for the 'busy' line to indicate a track is playing momentarily (I believe as it is located in the FAT table, but have no proof of this) and then to indicate it is stopped while the actual track is located and loaded. This can, if not considered, cause any software checking the busy status to imagine that the track started to play then, shortly afterwards came to an end. I have now had to check that the busy status indicates play continuously for a number of seconds before assuming the track requested is playing, along with that variable delay.
After saying that, I have got a microSD filled with over 6000 tracks, initially spread across 25 folders, with 254 tracks in all folders apart from the last one. I'm now trying to move all the tracks into one 'MP3' folder to make managing them easier.
As I read the datasheet (which is open to much interpretation) tracks in a folder named MP3 must have a four digit numeric index, not a three digit. When I tried a three digit numbering system in the MP3 folder, it failed to work for me using the command $12 to call the playback.
Files do not seem to need to be purely numerically named. A file named 0001.mp3 is treated the same as a file named 0001_File one in the folder.mp3 which makes things a little easier.
Requesting data responses back from the module gives yet more confusion. Trying to request the track count where there are multiple folders (01, 02, 03 etc.) appears to return only Number of folders - 1, if there are differing numbers of tracks within the folders it is difficult to deduce the actual track count. In very brief testing, where there is only one folder (MP3) the number of tracks - 1 seems to be returned. Although as of yet, I've only got 510 tracks in that folder, I'm still waiting for the copy procedure to complete before I can test with the full 6000+ tracks.
All that being said, I think the SPE035 module is fabulous. Once the limitations are understood they are a great little unit. Using either good quality headphones or connecting the auxiliary/headphone outputs to a quality amplifier shows that the sound quality they produce is very good.