7540 datasheet(307/366 Pages) RENESAS

307 / 366 page

3-85

REJ09B0018-0200Z

Rev.2.00

Jun 21, 2004

7540 Group

APPENDIX

3.3 Notes on use

3.3.12 Notes on CPU mode register

(1) Switching method of CPU mode register after releasing reset

Switch the CPU mode register (CPUM) at the head of program after releasing reset in the following

method.

Fig. 3.3.4 Switching method of CPU mode register

(2) 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 order to prevent the dead-lock by error-writing (ex. program run-

away), these bits can be rewritten only once after releasing reset. 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 executed to bits 2 to 4, 6 and 7, bits

5, 1 and 0 are locked.

After releasing reset

Switch the oscillation mode

selection bit (bit 5 of CPUM)

Switch the clock division ratio

selection bits (bits 6 and 7 of CPUM)

Main routine

Start with an on-chip oscillator (Note)

An initial value is set as a ceramic

oscillation mode. When it is switched to an

RC oscillation, its oscillation starts.

Select 1/1, 1/2, 1/8 or on-chip oscillator.

Wait by on-chip oscillator operation

until establishment of oscillator clock

When using a ceramic oscillation, wait until

establlishment of oscillation from oscillation starts.

When using an RC oscillation, wait time is not

required basically (time to execute the instruction to

switch from an on-chip oscillator meets the

requirement).

Note. After releasing reset the operation starts by starting an on-chip oscillator automatically.

Do not use an on-chip oscillator at ordinary operation.

7540 Datasheet(PDF) 307 Page - Renesas Technology Corp