R2025S/D
Power-on Reset, Oscillation Halt Sensing, and Supply Voltage
Monitoring
• PON, /XST, and VDET
The power-on reset circuit is configured to reset control register1, 2, and clock adjustment register when VDD
power up from 0v. The oscillation halt sensing circuit is configured to record a halt on oscillation by 32.768-kHz
clock pulses. The supply voltage monitoring circuit is configured to record a drop in supply voltage below a
threshold voltage of 2.1 or 1.3v.
Each function has a monitor bit. I.e. the PON bit is for the power-on reset circuit, and /XST bit is for the
oscillation halt sensing circuit, and VDET is for the supply voltage monitoring circuit. PON and VDET bits are
activated to “H”. However, /XST bit is activated to “L”. The PON and VDET accept only the writing of 0, but /XST
accepts the writing of 0 and 1. The PON bit is set to 1, when VDD power-up from 0V, but VDET is set to 0, and
/XST is indefinite.
The functions of these three monitor bits are shown in the table below.
PON
/XST
Monitoring for the
oscillation halt sensing
function
D5 in Address Fh
Low
VDET
a drop in supply voltage
below a threshold
voltage of 2.1 or 1.3v
D6 in Address Fh
High
Function
Monitoring for the
power-on reset function
Address
Activated
When VDD
power up from
0v
D4 in Address Fh
High
1
indefinite
0
accept the
writing
0 only
Both 0 and 1
0 only
The relationship between the PON, /XST, and VDET is shown in the table below.
PON
/XST
VDET
Conditions of supply voltage and
oscillation
Condition of oscillator, and
back-up status
0
0
0
Halt on oscillation, but no drop in
VDD supply voltage below threshold
voltage
Halt on oscillation cause of
condensation etc.
0
0
0
1
0
1
1
*
1
0
1
*
Halt on oscillation and drop in VDD
supply voltage below threshold
voltage, but no drop to 0V
No drop in VDD supply voltage
below threshold voltage and no halt
in oscillation
Drop in VDD supply voltage below
threshold voltage and no halt on
oscillation
Halt on oscillation cause of drop in
back-up battery voltage
Normal condition
No halt on oscillation, but drop in
back-up battery voltage
Drop in supply voltage to 0v
Power-up from 0v,
31
RICOH [ RICOH ELECTRONICS DEVICES DIVISION ]
