ST AN1957 Application note
AN1957
APPLICATION NOTE
Microprocessor Supervisor Functions
Designers of microproce ssor systems have to achieve high r eliability, in the face of a large number of
threats to stabilit y or e ven c orr ec t f u nc tio ning (s uch as , v ol tage d ro ps , g li tch es , long ramp-up times, p ro grams stuck in endless loops, etc.).
Supervisor circuits, fro m STMicroelectronics, provide highly effective solutions for minimizing the risks of
system failure, and for ensuring the safe running of the system, at a low cost. The m embers of the ST
Supervisor family offer various combinations of functions.
This Application N ote describes the main Sup ervisor functions and fea tures, to help the user to understand their principl es and the advantages of us ing them, through the desc ription of waveforms, rec ommended values, and hardware hookup diagrams.
Overview of ST Supervisors
– Microprocesso r Sup ervi s ors
STM705, STM706, STM706T/S/R, STM707, STM708, STM708T/S/R, STM813L, STM706P,
STM6321L/M, STM6321T/S/R, STM6821L/M, STM6821T/S/R, STM6823L/M, STM6823T/S/R,
STM6824L/M, STM6824T/S/R, STM6825L/M, STM6825T/S/R
– Microprocessor Supervisors with Switchover
M40SZ100W, M40Z111, M40Z300W, STM690A, STM692A, STM690T/S/R, STM802L/M, STM802T/
S/R, STM703, STM704, STM704T/S/R, STM806T/S/R, STM805L, STM805T/S/R, STM804T/S/R,
STM817L/M, STM818L/M, STM819L/M, STM795T/S/R
– TIMEKEEPER™ Sup er vis or s
M41ST95W, M41ST87W, M41ST87Y, M41ST85W, M41ST85Y, M41T315V, M48T201V, M48T201Y,
M48T212V
This Application Note is dedicated to the Microprocessor Supervisor and Microprocessor Supervisor with
Switchover families.
1/22March 2005
AN1957 - APPLICATION NOTE
TABLE OF CONTENTS
Overview of ST Supervisors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Table 1. Supervisor Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
POWER-ON RESET AND LOW VOLTAGE DETECT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Block Diagram Showing the Supervisor Reset Feature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Figure 1. Supervisor Reset Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Reset Waveforms for the Microprocessor Supervisor Devices . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 2. Power-On Reset and Low Voltage Detect Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Table 2. Reset Timings for the STM703/704 Supervisor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Table 3. Reset Thresholds (V
Hardware Hookup for the STM703/704 Supervisor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 3. Hardware Hookup for the STM703/704 Supervisor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
POWER-FAIL COMPARATOR. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Block Diagram of a Power-Fail Comparator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 4. Block Diagram of a Power-Fail Comparator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Hardware Hookup for the STM692A Supervisor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Example calculation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Table 4. Recommended Resistances for some V
Figure 5. Hardware Hookup for the STM692A Supervisor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Example of Power-Fail Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Figure 6. Voltage Drop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Table 5. Power-Fail Values for all Microprocessor Supervisors (except
for the devices mentioned in Table 6.). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Table 6. Power-Fail Values for 3V Microproces sor Superv isor s
with Battery Switchover (STM690/704/802/804/805/806) . . . . . . . . . . . . . . . . . . . . . . . . 10
). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
RST
Voltages . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
TRIP
WATCHDOG TIMER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Figure 7. Logic Diagram of a Watchdog Timer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Figure 8. Watchdog Timer Input and Output Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Table 7. Watchdog Timer Time-out Value for the STM705 Supervisor. . . . . . . . . . . . . . . . . . . . . 11
Hardware Hookup for the STM705 Supervisor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Figure 9. Hardware Hookup for the STM705 Supervisor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
BATTERY SWITCHOVER. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Figure 10.Block Diagram of a Battery switchover Device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Switchover Waveforms for the STM806R Supervisor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Figure 11.Battery Switchover Waveforms for the STM806R Supervisor. . . . . . . . . . . . . . . . . . . . . 14
Figure 12.Switchover Waveforms with Hysteresis Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Table 8. Switchover Values for the STM806R Supervisor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Hardware Hookup for the STM806R Supervisor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Figure 13.Hardware Hookup for the STM806R Supervisor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Hardware Hookup for the STM795 Supervisor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Figure 14.Hardware Hookup for the STM795 Supervisor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2/22
AN1957 - APPLICATION NOTE
CHIP ENABLE GATING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Figure 15.Chip Enable Gating Block Diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Typical Waveforms for the STM818 Supervisor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Figure 16.Typical Waveforms for the STM818 Supervisor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Table 9. Typical values for the STM818 Supervisor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Hardware Hookup for the STM818 Supervisor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Figure 17.Hardware Hookup for the STM818 Supervisor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
BATTERY FRESHNESS SEAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Figure 18.Typical Waveforms for the STM817/818/819 Supervisors . . . . . . . . . . . . . . . . . . . . . . . 20
CONCLUSION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
REVISION HISTORY. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Table 10. Document Revision History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3/22
AN1957 - APPLICATION NOTE
Table 1. Supervisor Options
Active
3V or 5V
Supervisor
Battery
Switch
over
Watchdog
Input
Watchdog
output
Active
Low
(1)
ST
R
STM690T/S/R 3V !! ! !
STM690A
STM692A
STM703
STM704
(3)
(3)
(4)
(4)
5V !! ! !
5V !! ! !
5V !!!!
5V !!!!
STM704T/S/R 3V !!!!
STM705
STM706
(5)
(5)
5V !!! !!
5V !!! !!
STM706T/S/R 3V !!! !!
STM706P
(6)
3V !! !!!
STM707 5V !! ! !
STM708 5V !! ! !
STM708T/S/R 3V !! ! !
STM795T/S/R 3V !
(2)
!
STM802L/M 5V !! ! !
STM802T/S/R 3V !! ! !
STM804T/S/R
STM805T/S/T
(7)
(7)
3V !!
3V !!
STM805L 5V !! ! !
STM806T/S/R 3V !!!!
STM813L 5V !! !!!
STM817L/M 5V !! ! ! !
STM818L/M 5V !! ! !!
STM819L/M 5V !!!!!
Note: 1. Push-Pull Output (unless otherwise specified).
2. Open drain output.
3. STM690A has a typical Reset Threshold of 4.65V and STM692A has a typical Reset Threshold of 4.40V.
4. STM703 has a typical Reset Threshold of 4.65V an d ST M704 has a typical Reset Threshold of 4.40 V.
5. STM705 has a typical Reset Threshold of 4.63V an d ST M706 has a typical Reset Threshold of 4.38 V.
6. The STM706P is identical to the STM706R, except for i ts Re set output which is active High.
7. STM804T/S/R and STM805T/S/R have different minimum and maximum Reset Thresholds with V
datasheet).
High
RST
!
!
(1)
(2)
(2)
Manual
Reset
Input
Power-fail
Comparator
!
!
falling and VCC rising (see
CC
ChipEnable
Gating
!
Battery
Freshness
Seal
4/22
AN1957 - APPLICATION NOTE
POWER-ON RESET AND LOW VOLTAGE DET ECT
After system start-up, a certain period of time is required for the power supply voltage to stabilize. For this
reason, ST Supervisor devices generate a Reset pulse after power-up (the minimum pulse width is
t
= 140ms, see Table 2.). Over the t
rec
the registers are set to their default values. This function is called Power-On Reset (POR).
Some designers attempt to use RC circuits, instead of a Reset implementation, because it is cheaper. But
it is also unsafe and u nreliable. RC circu its are not suitable fo r use as professional devi ces in industr ial
environments (see REFERENCES section, AN1772).
Another major function is Low Voltage Detect (LVD), which detects power supply brownouts and glitches.
Whenever V
after VCC increases above the V
falls below the Reset thres hold (V
CC
rst
is guaranteed. Also, if the triggering event is a narrow glitch, an RC circuit will only generate a poor Reset,
which may lead to malfunct ioning of the mi croprocess or (failing t o load registers correctly, ex ecuting invalid instructions, processing incorrect data, etc.).
Some Supervi sor devi ces in clude a Manua l Reset i nput (M
device, to generate a Reset. T ypically, a low-c ost push-button s witch is conne cted to the Manual Res et
input, which allows the user to restart the pr ocessor witho ut turning off the power. No add itional co mponents are needed becaus e Sup er viso r de vi ce s al ready inc lu de a deb oun ce circ uit that fil ter s the nois e of
contact closure. This fun ction c an be us ed to de bug , to pe rf orm the final test of a process or, or to r est art
a processor that is locked. The Reset button is also useful in systems where the processor is never turned
off, even when the system is in Off mode. Some processor s inclu de an interna l Reset that operates cor rectly under stable power supply conditions, but usually has difficulties in handling voltage drops and transients as well as looser tolerances for V
period, during which reset is asserted, the clock is stabilized and
rec
), the Reset output is ass erted and remains so t
RST
rec
threshold. In the case of an RC circuit, no minimum Reset pulse width
R) that can be used by the user, or the external
. The use of an external Reset is therefore recommended.
rst
Block Diagram Showing the Supervisor Reset Feature
Figure 1 . illustrates the Power-On Reset, Low Vol tage Detect and Manu al Reset features. The Reset is
asserted if one of the following events occurs:
■ system start-up;
■ Brownout, voltage drop, significant transient or glitch, negative voltage spike etc. on the power supply
line;
■ Manual Reset.
Figure 1. Supervisor Reset Features
V
CC
RESET
Power-On Reset
Low V oltage Detect
RST
ai10104
Note: V
MR RST
is the supply voltage, MR i s the Manu al Reset input . RST and RST are Reset outputs. Supervi sor dev ices can have an ac tive-Lo w
CC
output (R
ST), an active-High output (RST) or both.
5/22
AN1957 - APPLICATION NOTE
Reset Waveforms for the Microprocessor Supervisor Devices
When the input vol tage l evel re aches th e Re set th reshold (V
holds the Reset output signal (RST
) Low for a minimum time of t
2.).
Switching the Manual Reset signal (M
long as MR
is kept Low, and returns High t
R) to Low, causes the RST signal to go Low. RST remains Low as
after MR has been released.
rec
All Microprocesso r Supervisor device s have glitch immunit y. That is, the minimum MR
quired to Reset the output is fixed. All shorter pulses are ignored.
The Supervisor also r eacts to vol tage drops, brow nouts a nd signific ant glitc hes. If the inpu t voltage fa lls
below V
, the Reset output is asserted.
RST
Note that some Supervisor devices have a Reset output that is active High (RST). They therefore have a
waveform that is the inversion of the one that is shown in Figure 2.
Figure 2. Power-On Reset and Low Voltage Detect Waveforms
) after system power-up, the Super visor
RST
before driving it High again (see Figure
rec
pulse width re-
CC
Power-up
t
rec
Manual Reset
t
MLRL
Manual Reset
Glitch Immunity
t
MLMH
t
rec
Voltage drop
t
rec
Note: V
V
V
RST
MR
RST
is the Reset threshold, see Table 3. for values.
RST
Table 2. Reset Timings for the STM703/704 Supervisor
Symbol Alt Parameter Value Unit
t
MLMH
t
MLRL
t
rec
t
MR
t
MRD
MR Pulse Width Min. 150 ns
MR to RST Output Delay Max. 250 ns
V
to RST High and MR High to RS T High
RST
Min. 140 ms
Manual Reset Glitch Immunity Typ. 100 ns
AI10105
Table 3. Reset Thresholds (V
Supervisor
RST
)
Reset threshold (V
RST
)
Min. Typ. Max.
STM706P/70xR 2.55 2.63 2.70 V
STM70xS 2.85 2.93 3.00 V
STM70xT 3.00 3.08 3.15 V
STM692A/704/706/708, 8xxM 4.25 4.40 4.50 V
STM690A/703/705/707, 8xxL 4.50 4.65 4.75 V
6/22
Unit
AN1957 - APPLICATION NOTE
Hardware Hookup for the STM703/704 Supervisor
In the example of Figure 3., the Reset output is asserted in three cases:
■ during system power-up, until V
■ after a V
■ by pressing the Manual Reset push-button (the Reset button should be held for at least for t
drop (VCC falls below V
CC
The Manual Reset input is not necessarily connected to a physical push button switch, it can also be connected to a peripheral, provided that a minimum MR
If the Res et cir cuit i s plac ed in a noi sy envi ronme nt, or if MR
to use an external 0.1µF capacitor, as shown in Figure 3.
The MR
input includes an internal pull-up resistor. So in applications where the MR input is not used, the
pin can be left unco nnected. The MR
drain output.
It is always appropria te to conne ct a deco upling c apacitor in parall el with the power su pply. Th e recommended value is 1µF.
Figure 3. Hardware Hookup for the STM703/704 Supervisor
is stabilized (VCC is greater than V
CC
)
RST
) for a duration of t
RST
rec
MLMH
pulse width of 150ns is ensured.
is driven from long cables, it is recommended
input can be driven with a T TL ou tpu t, a CMOS output, or an open
).
Microprocessor
V
GND
RST
CC
V
CC
C
1
1µF
STM703/704
1
OUT
V
2
CC
V
3
SS
PFI PF0
45
SO8/TSSOP8
V
BAT
RST
MR
8V
7
6
Reset
Button
C
2
0.1µF
AI10106
7/22
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