ST AN1957 Application note

AN1957

APPLICATION NOTE

Microprocessor Supervisor Functions

Designers of microprocessor systems have to achieve high reliability, in the face of a large number of threats to stability or even correct functioning (such as, voltage drops, glitches, long ramp-up times, programs stuck in endless loops, etc.).

Supervisor circuits, from 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 members of the ST Supervisor family offer various combinations of functions.

This Application Note describes the main Supervisor functions and features, to help the user to understand their principles and the advantages of using them, through the description of waveforms, recommended values, and hardware hookup diagrams.

Overview of ST Supervisors

–Microprocessor Supervisors

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™ Supervisors

M41ST95W, M41ST87W, M41ST87Y, M41ST85W, M41ST85Y, M41T315V, M48T201V, M48T201Y, M48T212V

This Application Note is dedicated to the Microprocessor Supervisor and Microprocessor Supervisor with Switchover families.

March 2005

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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 (VRST) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

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 VTRIP Voltages . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 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 Microprocessor Supervisors
	with Battery Switchover (STM690/704/802/804/805/806) . . . . . . . . . . . . . . . . . . . . . . . .	10

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

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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

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Table 1. Supervisor Options

3V or 5V

Battery

Watchdog

Active

Manual

Power-fail

Chip-

Battery

Switch

Low

High

Reset

Enable

Freshness

Supervisor

over

Input

output

(1)

RST(1)

Input

Comparator

Gating

Seal

RST

STM690T/S/R

STM690A(3)

STM692A(3)

STM703(4)

STM704(4)

STM704T/S/R

STM705(5)

STM706(5)

STM706T/S/R

STM706P(6)

STM707

STM708

STM708T/S/R

STM795T/S/R

!(2)

STM802L/M

STM802T/S/R

STM804T/S/R(7)

!(2)

STM805T/S/T(7)

!(2)

STM805L

STM806T/S/R

STM813L

STM817L/M

STM818L/M

STM819L/M

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 and STM704 has a typical Reset Threshold of 4.40V.

5.STM705 has a typical Reset Threshold of 4.63V and STM706 has a typical Reset Threshold of 4.38V.

6.The STM706P is identical to the STM706R, except for its Reset output which is active High.

7.STM804T/S/R and STM805T/S/R have different minimum and maximum Reset Thresholds with VCC falling and VCC rising (see datasheet).

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POWER-ON RESET AND LOW VOLTAGE DETECT

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 trec = 140ms, see Table 2.). Over the trec period, during which reset is asserted, the clock is stabilized and 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 unreliable. RC circuits are not suitable for use as professional devices in industrial environments (see REFERENCES section, AN1772).

Another major function is Low Voltage Detect (LVD), which detects power supply brownouts and glitches. Whenever VCC falls below the Reset threshold (VRST), the Reset output is asserted and remains so trec after VCC increases above the Vrst threshold. In the case of an RC circuit, no minimum Reset pulse width is guaranteed. Also, if the triggering event is a narrow glitch, an RC circuit will only generate a poor Reset, which may lead to malfunctioning of the microprocessor (failing to load registers correctly, executing invalid instructions, processing incorrect data, etc.).

Some Supervisor devices include a Manual Reset input (MR) that can be used by the user, or the external device, to generate a Reset. Typically, a low-cost push-button switch is connected to the Manual Reset input, which allows the user to restart the processor without turning off the power. No additional components are needed because Supervisor devices already include a debounce circuit that filters the noise of contact closure. This function can be used to debug, to perform the final test of a processor, or to restart 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 processors include an internal Reset that operates correctly under stable power supply conditions, but usually has difficulties in handling voltage drops and transients as well as looser tolerances for Vrst. The use of an external Reset is therefore recommended.

Block Diagram Showing the Supervisor Reset Feature

Figure 1. illustrates the Power-On Reset, Low Voltage Detect and Manual 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

	VCC

		RESET
MR		RESET	RST
MR	Power-On Reset		RST
	Power-On Reset
	Low Voltage Detect		RST
	Low Voltage Detect		RST

ai10104

Note: VCC is the supply voltage, MR is the Manual Reset input. RST and RST are Reset outputs. Supervisor devices can have an active-Low output (RST), an active-High output (RST) or both.

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Reset Waveforms for the Microprocessor Supervisor Devices

When the input voltage level reaches the Reset threshold (VRST) after system power-up, the Supervisor holds the Reset output signal (RST) Low for a minimum time of trec before driving it High again (see Figure 2.).

Switching the Manual Reset signal (MR) to Low, causes the RST signal to go Low. RST remains Low as long as MR is kept Low, and returns High trec after MR has been released.

All Microprocessor Supervisor devices have glitch immunity. That is, the minimum MR pulse width required to Reset the output is fixed. All shorter pulses are ignored.

The Supervisor also reacts to voltage drops, brownouts and significant glitches. If the input voltage falls below VRST, the Reset output is asserted.

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

Power-up
VCC			Voltage drop
VRST
MR	Manual Reset
MR
	tMLMH
trec	tMLRL	trec	trec
RST
	Manual Reset
	Glitch Immunity

AI10105

Note: VRST is the Reset threshold, see Table 3. for values.

Table 2. Reset Timings for the STM703/704 Supervisor

Symbol

Alt

Parameter

Value

Unit

tMLMH

Pulse Width

Min.

150

tMLRL

Output Delay

Max.

250

RST

MRD

trec

VRST to

High and

High to

High

RST

Min.

140

Manual Reset Glitch Immunity

Typ.

100

Table 3. Reset Thresholds (VRST)

Supervisor		Reset threshold (VRST)		Unit
Supervisor	Min.	Typ.	Max.	Unit
	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

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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 VCC is stabilized (VCC is greater than VRST) for a duration of trec

■after a VCC drop (VCC falls below VRST)

■by pressing the Manual Reset push-button (the Reset button should be held for at least for tMLMH). 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 pulse width of 150ns is ensured.

If the Reset circuit is placed in a noisy environment, or if MR is driven from long cables, it is recommended 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 unconnected. The MR input can be driven with a TTL output, a CMOS output, or an open drain output.

It is always appropriate to connect a decoupling capacitor in parallel with the power supply. The recommended value is 1µF.

Figure 3. Hardware Hookup for the STM703/704 Supervisor

Microprocessor

VCC

STM703/704

VBAT

Reset

VOUT

VCC

Button

GND

VCC

RST

VSS

PFI

PF0

1µF

0.1µF

SO8/TSSOP8

RST

AI10106

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