M37544 Datasheet, PDF (65/68 Pages) Renesas Technology Corp – SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER

7544 Group
Notes on Clock Generating Circuit
1. Switch of ceramic/quartz-crystal oscillation and RC oscillation
After releasing reset, the oscillation mode selection bit (bit 5 of
CPU mode register (address 3B16)) is “0” (ceramic/quartz-crystal
oscillation selected). When the RC oscillation is used, after releas-
ing reset, set this bit to “1”.
2. Double-speed mode
The double-speed mode can be used only when a ceramic oscilla-
tion is selected. Do not use it when an RC oscillation is selected.
3. CPU mode register
Oscillation mode selection bit (bit 5), processor mode bits (bits 1
and 0) of CPU mode register (address 3B16) are used to select os-
cillation mode and to control operation modes of the
microcomputer. In order to prevent the dead-lock by erroneously
writing (ex. program run-away), these bits can be rewritten only
once after releasing reset. After rewriting, it is disabled to write any
data to the bit. (The emulator MCU “M37542RSS” is excluded.)
Also, when the read-modify-write instructions (SEB, CLB, etc.) are
executed to bits 2 to 4, 6 and 7, bits 5, 1 and 0 are locked.
4. Clock division ratio, XIN oscillation control, on-chip oscillator
control
The state transition shown in Fig. 81 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. 81.
5. On-chip oscillator operation
When the MCU operates by the on-chip oscillator for the main
clock, connect XIN pin to VCC through a 1 kΩ to 10 kΩ resistor and
leave XOUT pin open.
The clock frequency of the on-chip oscillator depends on the sup-
ply voltage and the operation temperature range.
Be careful that this margin of frequencies when designing applica-
tion products.
6. Ceramic resonator
When the ceramic resonator/quartz-crystal oscillation is used for
the main clock, connect the ceramic resonator and the external
circuit to pins XIN and XOUT at the shortest distance. A feedback
resistor is built-in.
7. RC oscillation
When the RC oscillation is used for the main clock, connect the
XIN pin and XOUT pin to the external circuit of resistor R and the
capacitor C at the shortest distance.
The frequency is affected by a capacitor, a resistor and a micro-
computer.
So, set the constants within the range of the frequency limits.
8. External clock
When the external signal clock is used for the main clock, connect
the XIN pin to the clock source and leave XOUT pin open.
Select “0” (ceramic oscillation) to oscillation mode selection bit.
Notes on Oscillation Control
1. Oscillation stop detection circuit
(1) When the stop mode is used, set the oscillation stop detection
function to “invalid”.
(2) When the ceramic or RC oscillation is stopped (bit 4 of CPU
mode register (address 3B16)), set the oscillation stop detec-
tion function to “invalid”.
(3) The oscillation stop detection circuit is not included in the emu-
lator MCU “M37542RSS”.
2. Stop mode
(1) When the stop mode is used, set the oscillation stop detection
function to “invalid”.
(2) When the stop mode is used, set “0” (STP instruction enabled)
to the STP instruction disable bit of the watchdog timer control
register (bit 6 of watchdog timer control register (address
3916)).
(3) The oscillation stabilizing time after release of STP instruction
can be selected from “set automatically”/“not set automati-
cally” by the oscillation stabilizing time set bit after release of
the STP instruction (bit 0 of MISRG (address 3816)). When “0”
is set to this bit, “0116” is set to timer 1 and “FF16” is set to
prescaler 1 automatically at the execution of the STP instruc-
tion. When “1” is set to this bit, set the wait time to timer 1 and
prescaler 1 according to the oscillation stabilizing time of the
oscillation. Also, when timer 1 is used, set values again to
timer 1 and prescaler 1 after system is returned from the stop
mode.
(4) The STP instruction cannot be used when the on-chip oscilla-
tor is selected by the clock division ratio selection bits (bits 7
and 6 of CPU mode register (address 3B16)).
(5) When the stop mode is used, set the on-chip oscillator oscilla-
tion control bit (bit 3 of CPU mode register (address 3B16)) to
“1” (on-chip oscillator oscillation stop).
(6) Do not execute the STP instruction during the A/D conversion.
Rev.1.04 2004.06.08 page 65 of 66
REJ03B0012-0104Z