IC in a PS2 optical mouse

PS2 optical mouse - brand unknown.

The 12-pins IC (integrated to the camera) is an ATL A2188 for which I found no datasheet in the Web.

Checked the rotary encoder used for the scroll wheel: mechanical switches with detents.

After probing its terminals with a scope, I realized that each switch brings in turn, in ON / OFF sequence its respective pin to ground.

Also the 3 microswitches (left - right - wheel) do exactly the same, bringing their respective pins to 0V when pressed down.

My question: the pull up "resistors" inside the IC, what are they? Common resistors or active transistors?

Additional (not related to the above): in the waiting state, the LED is driven permanently with a 6% duty cycle. For full brightness DC becomes 100%. Frequency is 96 Hz.

The sole resistor used in this mouse goes in series between the +5V and the LED.

weak pullup resistors are common in digital chips such as MCUs etc.
they are not resistors really but transistor acting as current source (Widlar).
only fairly small resistors are implemented as resistors in silicon,
large values are not economical as they take too much real-estate.