7905 datasheet(49/565 Pages) RENESAS

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RESET

7905 Group User’s Manual Rev.1.0

3-4

3.1.3 Power-on reset

The following describes the operation of the microcomputer at power-on reset.

➀ After powered on, within the several ten cycles of fsys after the voltage level at pin Vcc meets the recommended

operating conditions with the voltage level at pin RESET = “L,” the microcomputer initializes pins; refer to

section “3.2 Pin state.”

➁ After the voltage level at pin RESET goes from “L” to “H,” the microcomputer initializes the CPU and

SFR area within a period of 8 to 9 cycles of fsys. (Contents of the internal RAM area become undefined;

refer to section “3.3 State of internal area.”)

➂ After ➁, the microcomputer performs “Internal processing sequence after reset.”; refer to section “3.4

Internal processing sequence after reset.”

➃ The microcomputer executes a program beginning with the address which has been set into the reset

vector addresses (addresses FFFE16 and FFFF16).

Fig. 3.1.4 Example of power-on reset circuit

3.1 Reset operation

VCC

OUT

GND

Delay

capacity

RESET

VCC

VSS

Ω

GND

5 V

M51957AL

M37905

27 k

Ω

10 k

Ω

✽

The delay time is about 11 ms when Cd = 0.033

µF.

≈ 0.34 ✕ Cd [µs], Cd: [pF]

Figure 3.1.3 shows the power-on reset conditions.

Figure 3.1.4 shows an example of a power-on reset

circuit.

After the voltage level at pin Vcc meets the

recommended operating conditions and the oscillator’s

operation is stabilized (See Figure 3.1.3.), apply “L”

level of voltage to pin RESET for 10

µs or more.

When an oscillator is used, the time required for

stabilizing oscillation depends on the oscillator. For

details, contact the oscillator manufacturer.

VCC

RESET

Powered on there

VCC level

0.2 VCC level

Oscillation stabilized

µs

XIN

0 V

Fig. 3.1.3 Power-on reset conditions

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