The DS2408 is sensitive to the slew rate of the power-on sequence and can inadvertently power up with a test mode feature enabled. The symptom of this mode is the P0 pin is stuck midway between Vcc and GND and cannot be affected by the normal ‘Channel Access Write’ command.
The following sequence will disable any test mode functions. Run this sequence after every power-on reset.
1-Wire Reset + Presence Pulse Write Test Match ROM command: 96 hex Write 64 bit ROM of DS2408 Write Exit Test Mode command: 3C hex 1-Wire Reset + Presence Pulse