## The mystery of control registers for FM6126A chips

  

Datasheet for this chis chip is in chineese and does not shed a light on what those two control regs are.

An excellent insight could be found here

https://github.com/hzeller/rpi-rgb-led-matrix/issues/746#issuecomment-453860510

  
  

So there are two regs - **REG1** and **REG1**,

one could be written with 12 clock pusles (and usually called reg12, dunno why :))

the other one could be written with 13 clock pusles (and usually called reg13, dunno why :))

  

So I've done some measurmens on power consumption while toggling bits of **REG1** and it looks that it could provide a fine grained brighness control over matrix.

There are 6 bits (6 to 11) giving an increased brighness (compared to all-zeroes) and 4 bits (2-5) giving decreased brighness!!!

  

So it seems that the most bright (and hungry for power) value is

bool REG1[16] = {0,0,0,0,0, 1,1,1,1,1,1, 0,0,0,0,0}; and not {0,1,1,1,1, 1,1,1,1,1,1, 1,1,1,1,1} as it is usually used.

I'm not sure about bit 1 - it is either not used or I was unable to measure it's influence to brightness/power.

  

Giving at least 10 bits of hardware brightness control opens pretty nice options for offloading. Should dig into this more deeper.

  

Here are some of the measurments I've took for 2 64x64 panels filled with white color - reg value and corresponding current drain in amps.

  

|REG1 |bit value|Current, amps |
|--|--|--|
|REG1| 0111111 00000| >5 amps|
|REG1| 0100010 00000| 3.890 amp|
|REG1| 0100000 00000| 3.885 amp|
|REG1| 0011110 00000| 3.640 amp|
|REG1| 0011100 00000| 3.620 amp|
|REG1| 0011000 00000| 3.240 amp|
|REG1| 0010010 00000| 2.520 amp|
|REG1| 0010001 00000| 2.518 amp|
|REG1| 0010001 10000| 2.493 amp|
|REG1| 0010000 00000| 2.490 amp|
|REG1| 0010000 11110| 2.214 amp|
|REG1| 0001100 00000| 2.120 amp|
|REG1| 0001000 00000| 1.750 amp|
|REG1| 0000100 00000| 1.375 amp|
|REG1| 0000010 00000| 1.000 amp|
|REG1| **0000000 00000**| 0.995 amp|
|REG1| 0000001 11111| 0.700 amp|
|REG1| 0000000 01111| 0.690 amp|
|REG1| 0000000 10000| 0.690 amp|
|REG1| 0000000 11110| 0.686 amp|


/Vortigont/