====== RTL8231 GPIO expander ======

The RTL8231 IC is a companion chip often used with RTL83xx SoCs to extend the number of available GPIOs or LED outputs. It can be controlled via SMI (I2C-like), MIIM, or as a shift register. It comes in an LQFP-48 package:

      |36      25| 
    - +----------+ -
   37 |          | 24
      | RTL8231  |
   48 | o        | 13
    - +----------+ -
      |1       12|


===== Strapping pins =====

^ Number ^ Name ^ Description ^
| 15 | SMI_DIS | Disable SMI/MIIM mode and run in shift-register mode if pulled high. |
| 42 | MOD[0]  | SMI/MIIM mode: pull high to run in MIIM mode, pull low for SMI (I2C) mode. |
| 37 | ADDR[0] | **MIIM mode**: ADDR[4:0] indicates the device address \\ **SMI mode**: ADDR[2:0] indicates the three lowest bits of the device address. If ADDR[3] is pulled low, 8-bit register addresses are used, if it is pulled high 16-bit wide registers addresses are used. |
| 38 | ADDR[1] | ::: |
| 39 | ADDR[2] | ::: |
| 40 | ADDR[3] | ::: |
| 41 | ADDR[4] | ::: |

===== References =====
  * [[https://github.com/libc0607/Realtek_switch_hacking/blob/files/RTL8231_Datasheet_1.2.pdf | Datasheet]]