Datasheet

104
AT89C51IC2
4301D–8051–02/08
Power-off Flag The power-off flag allows the user to distinguish between a “cold start” reset and a
“warm start” reset.
A cold start reset is the one induced by V
CC
switch-on. A warm start reset occurs while
V
CC
is still applied to the device and could be generated for example by an exit from
power-down.
The power-off flag (POF) is located in PCON register (Table 81). POF is set by hard-
ware when V
CC
rises from 0 to its nominal voltage. The POF can be set or cleared by
software allowing the user to determine the type of reset.
Table 81. PCON Register
PCON - Power Control Register (87h)
Reset Value = 00X1 0000b
Not bit addressable
7 6 5 4 3 2 1 0
SMOD1 SMOD0 - POF GF1 GF0 PD IDL
Bit
Number
Bit
Mnemonic Description
7 SMOD1
Serial port Mode bit 1
Set to select double baud rate in mode 1, 2 or 3.
6 SMOD0
Serial port Mode bit 0
Cleared to select SM0 bit in SCON register.
Set to select FE bit in SCON register.
5 -
Reserved
The value read from this bit is indeterminate. Do not set this bit.
4 POF
Power-Off Flag
Cleared to recognize next reset type.
Set by hardware when V
CC
rises from 0 to its nominal voltage. Can also be set by
software.
3 GF1
General purpose Flag
Cleared by user for general purpose usage.
Set by user for general purpose usage.
2 GF0
General purpose Flag
Cleared by user for general purpose usage.
Set by user for general purpose usage.
1 PD
Power-Down mode bit
Cleared by hardware when reset occurs.
Set to enter power-down mode.
0 IDL
Idle mode bit
Cleared by hardware when interrupt or reset occurs.
Set to enter idle mode.