Datasheet

71M6521DE/DH/FE Data Sheet
Page: 58 of 107 Rev 3
Wake on PB
If the part is in SLEEP or LCD mode, it can be awakened by a rising edge on the PB pin. This pin is normally pulled to
GND and can be pulled high by a push button depression. Before the PB signal rises, the MPU is in reset due to the
internal signal WAKE being low. When PB rises, WAKE rises and within three crystal cycles, the MPU begins to
execute. The MPU can determine whether the PB signal woke it up by checking the IE_PB flag.
For debouncing, the PB pin is monitored by a state machine operating from a 32Hz clock. This circuit will reject
between 31ms and 62ms of noise. Detection hardware will ignore all transitions after the initial rising edge. This will
continue until the MPU clears the IE_PB bit.
Figure 25: Wake Up Timing
Wake on Timer
If the part is in SLEEP or LCD mode, it can be awakened by the wake-up timer. Until this timer times out, the MPU is
in reset due to WAKE being low. When the wake-up timer times out, the WAKE signal rises and within three crystal
cycles, the MPU begins to execute. The MPU can determine whether the timer woke it by checking the AUTOWAKE
interrupt flag (IE_WAKE).
The wake-up timer begins timing when the part enters LCD or SLEEP mode. Its duration is controlled by
WAKE_PRD[2:0] and WAKE_RES. WAKE_RES selects a timer LSB of either 1 minute (WAKE_RES=1) or 2.5 seconds
(WAKE_RES=0). WAKE_PRD[2:0] selects a duration of from 1 to 7 LSBs.
The timer is armed by WAKE_ARM=1. It must be armed at least three RTC cycles before SLEEP or LCD_ONLY is
initiated. Setting WAKE_ARM presets the timer with the values in WAKE_RES and WAKE_PRD and readies the timer
to start when the MPU writes to SLEEP or LCD_ONLY. The timer is reset and disarmed whenever the MPU is awake.
Thus, if it is desired to wake the MPU periodically (every 5 seconds, for example) the timer must be rearmed every
time the MPU is awakened.
time
System
Power
(V3P3SYS)
MPU Mode
PLL_OK
15 CK32
cycles
WAKE
LCD
PB or wake-
up timer
BROWNOUT