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STM708ADS6E

5 V supervisor

STMicroelectronics 

STM705, STM706, STM707, STM708, STM813L

3

Operation

Operation

3.1

Reset output

The STM705/706/707/708/813L supervisor asserts a reset signal to the MCU whenever

VCC goes below the reset threshold (VRST), a watchdog time-out occurs (if WDO is tied to

MR), or when the push-button reset input (MR) is taken low. RST is guaranteed to be a logic

low (logic high for STM707/708/813L) for VCC < VRST down to VCC =1 V for TA = 0 °C to

85 °C.

During power-up, once VCC exceeds the reset threshold an internal timer keeps RST low for

the reset time-out period, trec. After this interval RST returns high.

If VCC drops below the reset threshold, RST goes low. Each time RST is asserted, it stays

low for at least the reset time-out period (trec). Any time VCC goes below the reset threshold

the internal timer clears. The reset timer starts when VCC returns above the reset threshold.

3.2

Push-button reset input

A logic low on MR asserts reset. Reset remains asserted while MR is low, and for trec (see

Figure 29) after it returns high. The MR input has an internal 40 Ω pull-up resistor, allowing it

to be left open if not used. This input can be driven with TTL/CMOS-logic levels or with

open-drain/ collector outputs. Connect a normally open momentary switch from MR to GND

to create a manual reset function; external debounce circuitry is not required. If MR is driven

from long cables or the device is used in a noisy environment, connect a 0.1 µF capacitor

from MR to GND to provide additional noise immunity. MR may float, or be tied to VCC when

not used.

3.3

Note:

Watchdog input (STM705/706/813L)

The watchdog timer can be used to detect an out-of-control MCU. If the MCU does not

toggle the Watchdog Input (WDI) within tWD (1.6 s), the reset is asserted. The internal 1.6s

timer is cleared by either:

1. a reset pulse, or

2. by toggling WDI (high-to-low or low-to-high), which can detect pulses as short as 50 ns.

If WDI is tied high or low, a reset pulse is triggered every 1.8 s (tWD + trec), if WDO is

connected to MR.

See Figure 30 for STM705/706/813L.

The timer remains cleared and does not count for as long as reset is asserted. As soon as

reset is released, the timer starts counting.

The watchdog function may be disabled by floating WDI or tri-stating the driver connected to

WDI. When tri-stated or disconnected, the maximum allowable leakage current is 10 µA and

the maximum allowable load capacitance is 200 pF.

Doc ID 10520 Rev 9

11/33

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