ZHCSIH0B December 2017 – July 2019 LP87702-Q1
Operation of the LP87702-Q1 watchdog is shown in Figure 14 for an example when ENx pin is used for controlling power sequence and ENx pin is active.
WDI is the watchdog function input pin and WD_RESET is the reset output . WDI pin needs to be pulsed within a certain timing window to avoid watchdog expiration. Minimum pulse width is 100 µs. Watchdog expiration always causes a reset pulse at WD_RESET output, otherwise device behavior after watchdog expiration is programmable. WD_RESET output polarity and mode, push-pull or open drain, are also programmable.
Watchdog default settings are read from OTP during device start-up. Default settings in WD_CTRL_1 and WD_CTRL_2 register can be over-written via I2C (as long as WD_LOCK bit is not set to 1). Writing WD_LOCK = 1 in WD_CTRL_2 register locks watchdog settings until NRST input is driven low, power cycle or register reset by SW_RESET.
Long open, close and open window periods are independently programmable as shown in Table 4. When long open or open window expires before WDI input is received, watchdog enters WD Reset state. Also when WDI is received during close window, watchdog enters WD Reset. Long open period can be extended by a I2C write to WD_CTRL_1 or WD_CTRL_2 register; register access initializes the long open counter and the long open period restarts (except in Stop mode).
LP87702-Q1 behavior after WD expiration is programmable :
Watchdog settings in WD_CTRL_1 and WD_CTRL_2 registers are locked by setting WD_LOCK bit. WD_SYSTEM_RESTART_FLAG and WD_RESET_CNTR_STATUS can be cleared even if WD_LOCK = 1.
Description above is for a case where ENx pin is used for controlling power sequence and ENx pin is active. Depending on OTP settings and ENx pin state watchdog behavior can be slightly different:
|WD_LONG_OPEN_TIME||OTP||00 - 200 ms
01 - 600 ms
10 - 2000 ms
11 - 5000 ms
|WD_CLOSE_TIME||OTP||00 - 10 ms
01 - 20 ms
10 - 50 ms
11 - 100 ms
|WD_OPEN_TIME||OTP||00 - 20 ms
01 - 100 ms
10 - 200 ms
11 - 600 ms
LP87702-Q1 supports option to disable watchdog. WD_DIS pin function is multiplexed with CLKIN/GPIO2 functions. Watchdog disable option can be selected by setting register bit WD_DIS_CTRL = 1. When WD_DIS_CTRL = 1, WD is disabled if CLKIN/GPIO2/WD_DIS pin is HIGH and enabled if CLKIN/GPIO2/WD_DIS pin is LOW. If WD_DIS_CTRL is toggled to disable and re-enable WD, WD starts from Long Open window after re-enabling.
Default for WD_DIS_CTRL is set in OTP. WD_DIS_CTRL value can be changed via I2C until WD settings are locked. When WD_LOCK is set to 1, WD is enabled regardless of WD_DIS_CTRL value. WD_DIS_CTRL bit is protected by write lock. Three consecutive codes have to be written to WD_DIS_UNLOCK_CODE to open WD_DIS_CTRL for write access.