ST92F124 datasheet(115/429 Pages) STMICROELECTRONICS

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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

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.

Case 4: NMI = 1 (NMI kept high during the 3rd

write instruction of the sequence), correct

STOP bit setting sequence

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.

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:

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 116.).

Case 6: A wake-up event on the external wake-

up lines occurs during the STOP bit setting se-

quence

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:

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).

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:

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).

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.

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.

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.

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).

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