Recover Chip Freescale MC68705S3 8-Bit Microcomputers

We can recover chip Freescale MC68705S3 8-Bit Microcomputers, please view the chip features for your reference:


2.1 VDD, VSS, and EVSS

Power is supplied to the MCU through VDD and VSS. VSS is the power supply, and

VSS is ground. EVSS, available on the 44-pin PLCC, is an additional ground pin that

must be grounded with VSS. The MCU operates from a single 5-volt (nominal) power

supply. Very fast signal transitions occur on the MCU pins. The short rise and fall times

place high, short duration current demands on the power supply. To prevent noise

problems, provide good power supply bypassing at the MCU. Also, use bypass capac-

itors that have good high-frequency characteristics and situate them as close to the

MCU as possible. Bypass requirements vary, depending on how heavily the MCU pins

are loaded.

2.2 Reset (RESET)

An active low bidirectional control signal, RESET, acts as an input to initialize the MCU

to a known startup state. It also acts as an open-drain output to indicate that an internal

failure has been detected in either the clock monitor or COP watchdog circuit. The

CPU distinguishes between internal and external reset conditions by sensing whether

the reset pin rises to a logic one in less than two E-clock cycles after a reset has oc-

curred. It is not advisable to connect an external resistor-capacitor (RC) power-up de-

lay circuit to the reset pin of M68HC11 devices because the circuit charge time

constant can cause the device to misinterpret the type of reset that occurred. Refer to

SECTION 5 RESETS AND INTERRUPTS for further information.

Figure 2-3 illustrates a reset circuit that uses an external switch. Use a low voltage

interrupt circuit, however, to prevent corruption of RAM.

2.3 Crystal Driver and External Clock Input (XTAL, EXTAL)

These two pins provide the interface for either a crystal or a CMOS compatible clock

to control the internal clock generator circuitry. The frequency applied to these pins is

four times higher than the desired E-clock rate.

The XTAL pin is normally left unterminated when an external CMOS compatible clock

input is connected to the EXTAL pin. However, a 10 kΩ to 100 kΩ load resistor con-

nected from XTAL to ground can be used to reduce RFI noise emission. The XTAL

output is normally intended to drive only a crystal. The XTAL output can be buffered

with a high impedance buffer, or it can be used to drive the EXTAL input of another


In all cases, use caution around the oscillator pins. Load capacitances shown in the

oscillator circuits include all stray layout capacitances. Refer to Figure 2-4, Figure 2-

5, and Figure 2-6.

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