M37544 Datasheet, PDF (64/68 Pages) Renesas Technology Corp – SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER

7544 Group
4. I/O pin function when serial I/O is enabled.
The pin functions of P12/SCLK and P13/SRDY are switched to as
follows according to the setting values of a serial I/O mode selec-
tion bit (bit 6 of serial I/O control register (address 1A16)) and a
serial I/O synchronous clock selection bit (bit 1 of serial I/O control
register).
(1) Serial I/O mode selection bit → “1” :
Clock synchronous type serial I/O is selected.
• Setup of a serial I/O synchronous clock selection bit
“0” : P12 pin turns into an output pin of a synchronous clock.
“1” : P12 pin turns into an input pin of a synchronous clock.
• Setup of a SRDY output enable bit (SRDY)
“0” : P13 pin can be used as a normal I/O pin.
“1” : P13 pin turns into a SRDY output pin.
(2) Serial I/O mode selection bit → “0” :
Clock asynchronous (UART) type serial I/O is selected.
• Setup of a serial I/O synchronous clock selection bit
“0”: P12 pin can be used as a normal I/O pin.
“1”: P12 pin turns into an input pin of an external clock.
• When clock asynchronous (UART) type serial I/O is selected, it
functions P13 pin. It can be used as a normal I/O pin.
Notes on A/D conversion
1. Analog input pin
In order to execute the A/D conversion correctly, to complete the
charge to an internal capacitor within the specified time is re-
quired. The maximum output impedance of the analog input
source required to complete the charge to a capacitor within the
specified time is as follows;
About 35 kΩ (at f(XIN) = 8 MHz)
When the maximum output impedance exceeds the above value,
equip an analog input pin with an external capacitor of 0.01µF to
1µF between an analog input pin and VSS.
Further, be sure to verify the operation of application products on
the user side.
<Reason>
An analog input pin includes the capacitor for analog voltage com-
parison. Accordingly, when signals from signal source with high
impedance are input to an analog input pin, charge and discharge
noise generates. This may cause the A/D conversion/comparison
precision to be worse.
2. Clock frequency during A/D conversion
The comparator consists of a capacity coupling, and a charge of
the capacity will be lost if the clock frequency is too low. This may
cause the A/D conversion precision to be worse. Accordingly, set
f(XIN) in order that the A/D conversion clock is 500 kHz or over
during A/D conversion.
3. A/D conversion accuracy
As for AD translation accuracy, on the following operating condi-
tions, accuracy may become low.
(1) Since the analog circuit inside a microcomputer becomes sen-
sitive to noise when VREF voltage is set up lower than Vcc
voltage, accuracy may become low rather than the case
where VREF voltage and Vcc voltage are set up to the same
value.
(2) When VREF voltage is lower than [ 3.0 V ], the accuracy at the
low temperature may become extremely low compared with
that at room temperature. When the system would be used at
low temperature, the use at VREF=3.0 V or more is recom-
mended.
Notes on Watchdog Timer
1. The watchdog timer is operating during the wait mode. Write
data to the watchdog timer control register to prevent timer un-
derflow.
2. The watchdog timer stops during the stop mode. However, the
watchdog timer is running during the oscillation stabilizing time
after the STP instruction is released. In order to avoid the un-
derflow of the watchdog timer, the watchdog timer control
register must be written just before executing the STP instruc-
tion.
3. The STP instruction disable bit (bit 6 of watchdog timer control
register (address 3916)) can be set to “1” but cannot be set to
“0” by program.
Notes on RESET pin
1. Connecting capacitor
In case where the RESET signal rise time is long, connect a ce-
ramic capacitor or others across the RESET pin and the Vss pin.
And use a 1000 pF or more capacitor for high frequency use.
When connecting the capacitor, note the following :
• Make the length of the wiring which is connected to a capacitor
as short as possible.
• Be sure to verify the operation of application products on the
user side.
<Reason>
If the several nanosecond or several ten nanosecond impulse
noise enters the RESET pin, it may cause a microcomputer fail-
ure.
Rev.1.04 2004.06.08 page 64 of 66
REJ03B0012-0104Z