Renesas R8C/25 Manual descargar pdf (Pagina 3)

Rev. 1.1

R8C25 QuickDesign Guide.doc Page 3 of 10 12/23/2007

When using the POR feature do not connect a capacitor from the Reset input to

Ground or you may have conditions where the POR circuit does not operate

properly

3.2. Using RC Reset circuit

If the POR feature cannot be used, or is not desired, the MCU can be reliably

Reset using an RC circuit connected to the Reset input. In this case the POR

feature should be disabled by modifying the setting in the OFS register since the

default setting enables the POR feature.

When using the RC circuit the time constant must keep the voltage on the Reset

pin <0.2 X Vcc Min (0.44 volts) for a time equal to the power supply

stabilization time + 20 clock cycles of the slow speed internal oscillator. The

timing requirement starts after the Vcc has reached the minimum operating

voltage. Table 20.12 list the power supply stabilization time td(P-R) as 2 mSec

max.

Assuming that the ramp time on the Vcc is negligible the following calculation

will provide values that can provide the required delay.

The calculation uses the following parameters for approximating values required

• 2.2 mSec time minimum delay for margin

• Step Vcc to 5.5 VDC lower step voltages require less time constant

• Negligible rise time for power supply. When power supply ramp times

are greater than (xxx) the RC reset method can be difficult to apply

RC = - td(P-R) / ln (1- Vreset[max]/Vcc)

-2.2 mSec / ln (1- (.44V/5.5V))

RC ≈ 27 mSec

Though any resistor and capacitor values could be used that would provide the

required delay a few guidelines should be considered:

1. Higher resistor values increase noise susceptibility concerns

2. For noise and stability ceramic capacitor values are desirable

3. Large values of capacitance will store higher energy levels which must

be discharged to pull the reset line low

Based on these guidelines, typical values of resistance between 27k and 100k are

reasonable values. These resistor values allow using readily available ceramic

capacitors.

A diode is shown in parallel with the resistor in the Hardware manual. This diode

provides the capacitor with a low resistance discharge path when the line is to be

pulled low. Without this diode the capacitor will discharge through the internal

parasitic diode of the Reset pin, this can cause problems if the energy stored in the

capacitor is large.

3.3. Using External Reset circuit

If an external reset IC is desired and an on-chip debugger, for example the E8a,

will be used the Reset IC should have an open-drain output so the debugger can

1 2 3 4 5 6 7 8 9 10

Comentarios a estos manuales

Sin comentarios

Renesas R8C/25 Manual Pagina 3

Comentarios a estos manuales

Relacionado con productos y manuales para No Renesas R8C/25