Renesas R8C/25 Manual descarregar pdf (Página 2)

Rev. 1.1

R8C25 QuickDesign Guide.doc Page 2 of 10 12/23/2007

1. Power Supplies and Bypassing

1.1. Operating Frequency

The maximum operating frequency for the R8C/25 is dependent on the power

supply voltage supplied to the R8C. The electrical characteristics section of the

hardware manual defines the maximum frequency vs. voltage.

Input Voltage Maximum Operating Frequency

3.0 VDC ≤ Vcc ≤ 5.5VDC 20 MHz

2.7 VDC ≤ Vcc < 3.0 VDC 10 MHz

2.2 VDC ≤ Vcc < 2.7 VDC 5 MHz

1.2 Bypassing

Bypass recommendations are listed in the usage notes and the

AD Converter section of the Hardware manual. The following are minimum

bypass recommendations

• 0.1 uF ceramic from Vcc to Vss

• 0.1 uF ceramic from Vref to AVss (when using ADC)

•

All bypass capacitors should be connected using the shortest trace length possible

and thick traces. Though not specifically required a 100 pF capacitor from analog

input pins to AVss may help A/D converter noise rejection, this is discussed in

more detail in the ADC section of this guide

2. Memory

Some R8C/25 devices have memory that exceeds the 64K address boundary. For

application tips associated with utilizing this memory please see the Application Note

Using R8C Expanded Memory.

3. Reset Requirements and the Reset Circuit

The R8C/25 has a Power-On-Reset (POR) circuit. This circuit provides a simple means

to provide a stable reset signal to the MCU without the need for an external Reset IC.

The POR circuit of the MCU should be sufficient for most designs; however, the POR

only provides an internal Reset signal so there must be a system consideration if a System

Reset signal is required. Under these conditions an MCU port pin can be used to provide

that signal see Application Brief ********

3.1. Using the POR feature

Section 5.2 of the Hardware manual describes using the POR feature of the device.

To use this feature a 5K (approximate value) resistor is connected between Vcc

and the Reset pin. That is all that is required to use the POR feature.

To use the POR feature the Vcc must rise above 2.4V to release the Reset signal.

The voltage detection circuit that is used for the POR asserts at 2.2V, which is the

minimum operating voltage of the device, and does not release until the voltage

rises above 2.4V. (See electrical characteristics Voltage Detection 0 and Power

On Reset, Voltage Monitor 0 sections for full characteristics)

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