7540 datasheet(71/366 Pages) RENESAS

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1-52

REJ09B0018-0200Z

Rev.2.00

Jun 21, 2004

HARDWARE

7540 Group

s Notes on clock generating circuit

For use with the oscillation stabilization set bit after release of the

STP instruction set to “1”, set values in timer 1 and prescaler 1 af-

ter fully appreciating the oscillation stabilization time of the

oscillator to be used.

• Switch of ceramic and RC oscillations

After releasing reset the operation starts by starting an on-chip os-

cillator. Then, a ceramic oscillation or an RC oscillation is selected

by setting bit 5 of the CPU mode register.

• Double-speed mode

When a ceramic oscillation is selected, a double-speed mode can

be used. Do not use it when an RC oscillation is selected.

• CPU mode register

Bits 5, 1 and 0 of CPU mode register are used to select oscillation

mode and to control operation modes of the microcomputer. In or-

der to prevent the dead-lock by error-writing (ex. program

run-away), these bits can be rewritten only once after releasing re-

set. After rewriting it is disable to write any data to the bit. (The

emulator MCU “M37540RSS” is excluded.)

Also, when the read-modify-write instructions (SEB, CLB) are ex-

ecuted to bits 2 to 4, 6 and 7, bits 5, 1 and 0 are locked.

• Clock division ratio, XIN oscillation control, on-chip oscillator control

The state transition shown in Fig. 53 can be performed by setting

the clock division ratio selection bits (bits 7 and 6), XIN oscillation

control bit (bit 4), on-chip oscillator oscillation control bit (bit 3) of

CPU mode register. Be careful of notes on use in Fig. 53.

• On-chip oscillator operation

The clock frequency of the on-chip oscillator depends on the sup-

ply voltage and the operation temperature range.

Be careful that variable frequencies when designing application

products.

s Electric Characteristic Differences Among

Mask ROM and One TIme PROM Version

MCUs

There are differences in electric characteristics, operation margin,

noise immunity, and noise radiation among mask ROM and One

Time PROM version MCUs due to the differences in the manufac-

turing processes.

When manufacturing an application system with One Time PROM

version and then switching to use of the mask ROM version, per-

form sufficient evaluations for the commercial samples of the

mask ROM version.

NOTES ON PERIPHERAL FUNCTIONS

s Note on Power Source Voltage

When the power source voltage value of a microcomputer is less

than the value which is indicated as the recommended operating

conditions, the microcomputer does not operate normally and may

perform unstable operation.

In a system where the power source voltage drops slowly when

the power source voltage drops or the power supply is turned off,

reset a microcomputer when the supply voltage is less than the

recommended operating conditions and design a system not to

cause errors to the system by this unstable operation.

7540 Datasheet(PDF) 71 Page - Renesas Technology Corp

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