Day 9
Using The Serial Interface
The Serial Interface (a.k.a The SI Registers) is used for many things including reading controller
data and writing to mempaks and using the Rumble. I recommend that you see LaC's hardware doc for the best
info on using the SI.
The Info
The SI does DMA with a 64 byte command packet between PIH RAM and main memory (PIH RAM is in main
memory, but it's like taboo to use normal writes with it...). The 64b command packet can be visualized like
this (all diagrams adapted/copied from LaC's doc):
[64byte block] at 0xbfc007c0 (1fc007c0)
{ Command : Results placed here when you DMA the data back to main memory
ff 00 00 00 : 00 00 00 00 - 8 bytes
ff 00 00 00 : 00 00 00 00 - 8 bytes
ff 00 00 00 : 00 00 00 00 - 8 bytes
ff 00 00 00 : 00 00 00 00 - 8 bytes
ff 00 00 00 : 00 00 00 00 - 8 bytes
ff 00 00 00 : 00 00 00 00 - 8 bytes
ff 00 00 00 : 00 00 00 00 - 8 bytes
ff 00 00 00 : 00 00 00 01 - 8 bytes
} ^^pif status/control byte
It will help for our purposes that the last byte, refered to by LaC as the control byte, always be 01.
Although there's room for 4 bytes, most commands only use 3, so put the Reset command ($FF) in the first spot
to align everything so you get the Results conveniently 4 byte aligned.
This diagram describes the form of the 3 bytes of the command:
Command Types:
| Command | Description |t |r |
+---------+--------------------------+-----+
| 00 | request info (status) |01|03|
| 01 | read button values |01|04|
| 02 | read from mempack slot |03|21|
| 03 | write to mempack slot |23|01|
| 04 | read eeprom |02|08|
| 05 | write eeprom |10|01|
| ff | reset |01|03|
NOTE: values are in hex
t r Command
Every command is in the form of (referring to the diagram): t r Command. So reading a controller is 0xFF010401 (that
first 0xFF is the alignment thing from the first diagram.). The only thing that I understand how to do so far is read
the controllers, the command packet for reading the controllers looks like this:
[64byte block]
{ command data
ff 010401 - ffffffff
ff 010401 - ffffffff
ff 010401 - ffffffff
ff 010401 - ffffffff
fe 000000 - 00000000
00 000000 - 00000000
00 000000 - 00000000
00 000000 - 00000001
}
The byte 0xFE means that there aren't any more commands, it can be left off if the packet is totally full of commands.
Appearantly, the controller that we are reading changes by 1 with each controller read command, so the first one is
controller 1, then #2,#3,and #4. Also, if you just read 1 controller, the reading resets between sending command packets,
so you can't read each controller 1 at a time with 1 read command in each packet (I don't know why you'd do that, but you
can't anyway...).
Note that some commands aren't only 3 bytes long, that's because of the real meaning of t and r. t is how many bytes
we are sending in this specific command not including the r byte (but including, appearantly, the Command byte that
comes after r ). r is how many bytes we want returned as Results from the PIH. Also note that having more than 3
bytes in the specific command makes the previous type of diagram out of alignment with where the results are going to
end up. Results will always end up after the specific command, they just might not be perfectly aligned on those 4
bytes...
This Day In Review
Most (more like all) of this information was edited from LaC's hardware document v0.8.
Please note that this was just to explain SI (or atleast how I understand it, might not be correct.) and
doing specific things like reading controllers and making the RumblePak go will be covered in separate "Day"s.
Read The Docs!,
- GBAGuy
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