The next thing to look at is how the display deals with the above three modes. Although we wont be using these modes they do show how we may be able to get a few extra bits that we can use.
Radio mode is actually REALLY simple. For some reason though my head unit wont stay in AM mode so I wont cver it but it should be pretty similar. I susect there is a variation on the tuning mode that will display the right steps. I also dont have the telltale codes as I cant actually see them on my display 🙁
We are interested in the code 0x09A, 0x02, 0xaa, 0xbb. This seems to put the display in frequency mode and then displays the frequency in steps of .1MHz from 87.5 so for example 0x01 would be 87.6Mhz. 0xbb is always set as zero but it may be this is used for AM mode.
0x9A, 0x13 is issued just before, I dont think this is mode switching but likeley refers to setting of the telltales. It does seem this is used with every LCD mode change however I have noticed the micro does update the screen whenever it can rather than when needed.
Now the fun (and useful) one. RDS mode. This seems just as simple as above. On switching from frequency mode to RDS mode we see the following commands…
0x9A, 0x02, 0xaa, 0x00 – Freq display refresh, not sure why this is sent
0x9A, 0x23, 0x00, 0x00, 0x00 – Clear display
Why we are updating the frequency then clearing the display I really dont know. But once the display is clear the head unit sends the station ID as text. The bytes 0x9A and 0x48 are followed by 8 characters as their ASCII codes. If the ID is less then it is padded with 0x20 (space). Exactly what characters are valid is unknown. It should be possible to implement scrolling though as the display updates very fast. It may be possible to skip the clear to make it smoother.
Next: Tape Mode