the main usb i/o chip can send asynchronous serial data to other chips

16 adresses for slaves
max 16 data bytes per packet

P is a parity bit belonging to it's byte

packet format:

#	8 7 6 5 4 3 2 1 0
0	P|  ADDR | SIZE-1
1	P|    DATA1
2	P|    DATA2
.
n	P|    DATAn 
n+1	P| XOR of DATA[1-n] (starting with b'11111111')

since it makes no sense to send packets with no
data, we store SIZE-1, this allows packet sizes
from 1 to 16 inclusive

the ping-pong buffers live at 0x20 and 0x30 in bank0

variables 
	buf0 16 bytes
	buf1 16 bytes
	buf_size <- used by receiver
	buf_len <- used during processing
	address A A A A X X X X, address of this device 4 bits
	FSR_tmp
	usartrstate 7 6 5 4 3 2 1 0
                    A P X X X S S S
A: buffer is available for processing, it should be 
   cleared if processing is finished
P: ping-pong state (which buffer belongs to the receiver...)
X: unused
S: state
	FSR         0 0 1 H L L L L
H buffer wherin would be written if there were something to be written
L address within buffer

Receiver:

- byte enters through UART...
- check OERR
  * if OERR = 1: clear error, ignore byte, set state0
- check FERR
  * if FERR = 1:              ignore byte, set state0
- check parity
  * if FAIL :                 ignore byte, set state0
- check timeout
  * if timeout = 1	                   set state0, continue
- reset timeout
- call usart_state

receiver state machine
~~~~~~~~~~~~~~~~~~~~~~
state0:  'address'
- set buf_size, FSR &= 11110000, buf_xor = 0xFF
- match address
  * if MATCH set state1
  * else set state3 (increment buf_size twice)

state1: 'data'
- store byte at INDF, xor with buf_xor
- match lowest 4 bits of buf_size with same bits of FSR
  * if MATCH switch to state2

state2: 'checksum'
- compare byte with buf_xor
  * if EQUAL // transfer is finished
    - is the other buffer free? (if no, this buffer will be lost)
      * YES
        - flip P, set A, set FSR to other buffer
    - set state0

state3: 'throw bytes away that are not adressed to us'
  

dimmer protocol
~~~~~~~~~~~~~~~

addressed commands:

* 0EFFAAAA

EFF is command:
	G = 0 (respective value applies to each selected) ALWAYS 4 values
	    1 (one value applies to all selected)
	FF = 00 dim value
	     01 inc value
	     10	inc/dec + prescaler value 

AAAA is address of dimmers 3-0, so AAAA=1011 means: dim3,dim1 and dim0

* 10XXXDDD

DDD is dimmer codepage, may not all be zero

* 110XXXXD