R8C/38T-A Group
6. Resets
Under development Preliminary document
Specifications in this document are tentative and subject to change.
R01UH0241EJ0010 Rev.0.10
Page 56 of 730
Aug 05, 2011
6.3.4
Voltage Monitor 0 Reset
The voltage monitor 0 reset is due to the on-chip voltage detection 0 circuit. The voltage detection 0 circuit
monitors the voltage applied to the VCC pin. Vdet0 is the detection level. To use the voltage monitoring 0 reset,
set the LVDAS bit in the OFS register to 0 (voltage monitoring 0 reset enabled after reset). The Vdet0 detection
level can be changed by setting bits VDSEL0 and VDSEL1 in the OFS register.
When the input voltage to the VCC pin falls to the Vdet0 level or lower, the CPU, SFRs and I/O ports are
initialized.
When the voltage applied to the VCC pin next rises to the Vdet0 level or higher, the low-speed on-chip
oscillator clock count starts. When the low-speed on-chip oscillator clock count reaches 32, the internal reset
signal goes high and the MCU proceeds to the reset sequence (refer to
Figure 6.2). The low-speed on-chip
oscillator clock with no division is automatically selected as the CPU clock after a reset.
To use the power-on reset, set the LVDAS bit and enable the voltage monitor 0 reset.
Bits VDSEL0, VDSEL1, and LVDAS cannot be changed by a program. To set these bits, write values to b4 to
b6 at address 0FFFFh using a flash programmer.
For details on the states of the SFRs after a voltage monitor 0 reset, refer to 3.2 Special Function Registers
(SFRs).
The internal RAM is not initialized. If the voltage applied to the VCC pin falls to Vdet0 or lower while writing
to the internal RAM, the RAM values will be undefined.
Figure 6.6
Voltage Monitor 0 Reset Circuit Example and Operation
Notes:
1. Vdet0 indicates the voltage detection level of the voltage detection 0 circuit. For details, refer to the
Voltage Detection Circuit chapter.
2. To use the power-on reset, enable the voltage monitor 0 reset by setting the LVDAS bit in the OFS
register to 0.
Voltage detection 0
circuit response time
Vdet0
0.5V
External power VCC
1
fLOCO
32
Internal reset signal