External RPD circuitry examples

AN2340

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2

External RPD circuitry examples

To ensure that both functions explained in the previous chapter work correctly, external

circuitry must be connected to the RPD.

2.1

RC network

A simple RC network can be connected to the RPD pin leading to correct behavior both

during system reset and return from power down. The cases will be analyzed separately

considering that the resistance R and the capacitor C are connected as in

Figure 3.

2.1.1

System reset

On power-up, the logical low level on the RPD pin forces an asynchronous hardware reset

when RSTIN is asserted low (see

Figure 1). The external pull-up R will then charge the

capacitor C. Note that an internal pull-down device on the RPD pin is turned on when the

RSTIN pin is low, and causes the external capacitor (C) to begin discharging at a typical rate

of 100 to 200

µA. With this mechanism, after a power-up reset, short low pulses applied on

RSTIN produce synchronous hardware reset. If RSTIN is asserted longer than the time

needed for C to be discharged by the internal pull-down device, then the device is forced

into an asynchronous reset.

2.1.2

Return from power down

To exit Power Down mode with external interrupt, an EXxIN pin must be asserted for at least

40ns (x = 7...0). This signal enables the internal main oscillator (if not already running) and

PLL circuitry, and also turns on the internal weak pull-down on RPD pin. The discharging of

the external capacitor C provides a delay that allows the oscillator and PLL circuits to

stabilize before the internal CPU and Peripheral clocks are enabled. When the voltage on

the RPD pin drops below the threshold voltage, the CPU and Peripheral clocks are enabled

and the device resumes code execution (see

Figure 2 on page 5).

Figure 3.

RPD pin: Internal (simplified) and external circuitry

+

C

R

RPD

Pull-Up

Weak Pull-Down

(~ 200

µA)

Driven by internal

reset/pwrnd

circuitry