MM1145 查看數據表（PDF） - Mitsumi

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MM1145

System Reset (with built-in watchdog timer)

Mitsumi 

MITSUMI

System Reset (with built-in watchdog timer) MM1145

Formula for CT Pin External Constant

Watchdog timer monitoring time (TWD), watchdog timer reset time (TWR) and reset hold time (TPR) during power

supply rise can be changed by varying CT capacitance. TWD also can be changed with RRCT.

The variable times are expressed by the following.

1. MM1145A

TPR (ms) .=. 5000 CT (µF)

TWD (ms) .=. 2500 CT (µF) RRCT (MΩ)

TWR (ms) .=. 500 CT (µF)

Example : when CT=0.02µF, RRCT=1MΩ

TPR .=. 100ms

TWD .=. 50ms

TWR .=. 10ms

2. MM1145B

TPR (ms) .=. 2000 CT (µF)

TWD (ms) .=. 5500 CT (µF) RRCT (MΩ)

TWR (ms) .=. 500 CT (µF)

Example : when CT=0.02µF, RRCT=1MΩ

TPR .=. 40ms

TWD .=. 110ms

TWR .=. 10ms

Formula for Watchdog Timer Monitoring Time (TWD) Adjustment

The ratio between TPR and TWD can be changed within the range below by adjusting TWD with RRCT.

1. MM1145A

TWD/TPR .=. 2.0 RRCT (0.39 (MΩ) <= RRCT <= 2 (MΩ))

2. MM1145B

TWD/TPR .=. 2.75 RRCT (0.39 (MΩ) <= RRCT <= 2 (MΩ)

How to Use RCT Pin

Example 1

Example 2

1. Watchdog timer operates when RCT pin is open.

2. Watchdog timer stops operating when RCT pin is connected to ground.

Description of Operation

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1. RESET goes low when VCC rises to approximately 0.8V.

Approximately 1µA (VCC=0.8V) of pull up current is output from RESET.

2. Capacitor CT charging starts when VCC rises to VSH (.=. 4.3V). Output is in reset state at this time.

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3. Output reset is released (RESET goes high) after a certain time (TPR), from when CT starts charging until

discharge (the time from when CT voltage reaches a certain threshold value VT1 (.=. 1.4V) until CT voltage

drops to a certain threshold value VT2 (.=. 0.2V).

Reset hold time : TPR is as follows.

TPR (ms) .=. 5000 CT (µF)

CT charging starts again after reset release, and watchdog timer operation begins.

Clock input during power ON reset time (TPR) will cause mis-operation.

4. If CK1 and CK2 are input in that order (or simultaneously) during CT charging, CK2 negative edge trigger

causes the TC pin to switch from charging to discharge.

5. Discharge switches to charging when CT voltage drops to threshold value VT2 (.=. 0.2V). Steps 4 and 5 are

repeated while a normal clock is input from the logic system.

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