Engineering Bulletin

External Components Attached to the RESET Pin

EB413

MOTOROLA

3

In this case, the internal reset circuit of the HC05 device will hold reset

low internally for the first 4064 system clock cycles (2 ms at 4-MHz

clock). Thus, the internal reset will be released between 7 ms and 12 ms

after power on. This arrangement simply requires a 10-k

Ω pullup resistor

externally on the RESET pin. Some HC05 devices have an internal

resistor which means that the RESET pin may be left unconnected or

simply have an external capacitor.

In a situation where the power supply takes more than 10 ms to reach

4 V, the oscillator is likely to have started up and the internal reset

particular clock frequency used in the application. This could cause

incorrect device operation and a fatal error for the application is likely. In

this case an external delay is necessary on the RESET pin.

The simplest delay is a resistor/capacitor (RC) network (see Figure 1)

where the capacitor is charged up by the current flowing through the

resistor until it reaches a level detected by the HC05 RESET pin circuitry

as a logic 1 state. This value is normally defined as a minimum required

RESET pin does have a Schmitt input but the hysteresis can be as little

as 300 mV on some devices. The diode ensures that the capacitor

could keep the MCU powered up for a short time, especially if the MCU

is in stop mode where power consumption is very low.

Figure 1. RC Network to Delay Reset Release

by Approximately 10 ms

D1

+5 V

47 k

Ω

220 nF

MCU

RESET

RESET PIN

RESET RELEASED

RESET ACTIVE

Freescale Semiconductor, Inc.

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