Hi Yan
You are true the bit 2 and 3 of the Register ALERT and ALERT_MASK are not used so you don't care about them
for each of the 4 bits of the ALERT register that are useful , you can clear it by reading 1 register
the register bit 0 is ALERT on Phy event ( Phy is fully performed by STUSB4500 and this alert must remain masked ) so you don't care about this one , it give only the indication that messages was send or received if you want absolutely to clear it , simply read the register @0x17 PHY_STATUS
Protocol level is also fully managed by STUSB4500 , but to be able to take decision regarding power , you need to intercept SOURCE PDO during power negotiation
the bit 5 of the register 0x16 give you the indication that message is available in reception buffer , so you can read it , be careful that if in between a new message is received the buffer is over ,this operation must be perform by interrupt handler because it could be very short < 1.6 ms
please check my yesterday mail
for example
if you have mask register to 0x91 you have to check only 3 registers
if you have 0x02 in ALERT register read register 0x16 ( to know if a message was received ) for example Sources PDO
if you have 0x40 in ALERT register read register 0x0D( connection event )
if you have 0x20 in ALERT register read register 0xF0 (monitoring event )
by the way pull up is mandatory on ALERT pin #19
you need to use for that the interrupt handler ALARM_MANAGEMENT () combined with falling edge interrupt
the package STSW-STUSB003 help you to properly manage the STUSB4500 some bug correction and enhancement was added in last package
BR
MiBA
