ST92F124/F150/F250 - INTERRUPTS
WAKE-UP / INTERRUPT LINES MANAGEMENT UNIT (Cont’d)
Case 3: NMI = 1 (NMI kept high during the 3rd
write instruction of the sequence), bad STOP
bit setting sequence
STOP = 0, EX_STP = 0
The application can determine why the ST9 did
not enter STOP mode by polling the pending
bits of the external lines (at least one must be at
1).
The result is the same as Case 1:
STOP = 0, EX_STP = 0
This means that the ST9 did not enter STOP mode
due to a bad STOP bit setting sequence: the user
must retry the sequence.
2. Interrupt requests to CPU are enabled: in this
case the ST9 will not enter STOP mode and the
interrupt service routine will be executed. The
status of STOP and EX_STP bits will be again:
Case 4: NMI = 1 (NMI kept high during the 3rd
write instruction of the sequence), correct
STOP bit setting sequence
STOP = 0, EX_STP = 0
The interrupt service routine can determine why
the ST9 did not enter STOP mode by polling
the pending bits of the external lines (at least
one must be at 1).
In this case:
STOP = 1, EX_STP = 0
This means that the ST9 did not enter STOP mode
due to NMI being kept high. The user should clear
the STOP bit via software.
If the MCU really exits from STOP Mode, the
RCCU EX_STP bit is still set and must be reset by
software. Otherwise, if NMI was high or an Inter-
rupt/DMA request was acknowledged during the
STOP bit setting sequence, the RCCU EX_STP bit
is reset. This means that the MCU has filtered the
STOP Mode entry request.
Note: If NMI goes to 0 before resetting the STOP
bit, the ST9 will not enter STOP mode.
Case 5: A rising edge on the NMI pin occurs
during the STOP bit setting sequence.
The NMI interrupt will be acknowledged and the
ST9 will not enter STOP mode. This implies:
The WKUP-INT bit can be used by an interrupt
routine to detect and to distinguish events coming
from Interrupt Mode or from Wake-up Mode, allow-
ing the code to execute different procedures.
STOP = 0, EX_STP = 0
This means that the ST9 did not enter STOP mode
due to an NMI interrupt serviced during the STOP
bit setting sequence. At the end of NMI routine, the
user must re-enter the sequence: if NMI is still high
at the end of the sequence, the ST9 can not enter
STOP mode (See “NMI Pin Management” on
page 115.).
To exit STOP mode, it is sufficient that one of the
16 wake-up lines (not masked) generates an
event: the clock restarts after the delay needed for
the oscillator to restart.
The same effect is obtained when a rising edge is
detected on the NMI pin, which works as a 17th
wake-up line.
Case 6: A wake-up event on the external wake-
up lines occurs during the STOP bit setting se-
quence
Note: After exiting from STOP Mode, the software
can successfully reset the pending bits (edge sen-
sitive), even though the corresponding wake-up
line is still active (high or low, depending on the
Trigger Event register programming); the user
must poll the external pin status to detect and dis-
tinguish a short event from a long one (for example
keyboard input with keystrokes of varying length).
There are two possible cases:
1. Interrupt requests to the CPU are disabled: in
this case the ST9 will not enter STOP mode, no
interrupt service routine will be executed and
the program execution continues from the
instruction following the STOP bit setting
sequence. The status of STOP and EX_STP
bits will be again:
114/426
9
STMICROELECTRONICS [ ST ]
