Power-On Reset; Power-Saving Modes Of; Chmos Power Reduction Modes; Operatfon - Intel MCS 51 User Manual

i~o
HARDWARE DESCRIPTION OF THE 8051,8052 AND 80C51
Table 3. Reset Values of the SFRS
I
SFR Name Reset
Value
Pc OOOOH
ACC OOH
B OOH
Psw OOH
SP
07H
I
DPTR I OOOOH I
I
PO-P3 I FFH I
1P(8051) XXXOOOOOB
1P(8052) XXOOOOOOB
IE [8051) OXXOOOOOB
1
IE (8052) OXOOOOOOB
TMOD OOH
TCON
OOH
THO OOH
TLO OOH
TH1 OOH
I
TL1 I OOH I
[
TH2 (8052)
I
OOH
J
TL2 (8052) OOH
RCAP2H(8052) OOH
RCAP2L(8052) OOH
SCON OOH
SBUF
Indeterminate
PCON (HMOS) OXXXXXXXB
PCON (CHMOS) OXXXOOOOB
,.",l
k
UKIL
=
3
'cc
Sml
ST
Isa
270252-21
Figure25. PoweronResetCircuit
POWER-ONRESET
For HMOSdeviceswhenVCCis turned on an automat-
ic reset can be obtainedby connectingthe RST pin to
V~ througha 10pF capacitor and to Vss throughan
8.2 Kf2 reeistor (Figure 26). The CHMOSdeviceado
not require this resistor although its presencedoea no
harm. In fact, for CHMOSdevicesthe externalresistor
can be removedbecausethey havean internalpulldown
on the RST pin. The capacitor valuecould then be r-
duced to 1 pF.
Whenpoweris turned on, the circuit holdsthe RSTpin
highfor an amount of time that dependson the capaci-
tor value and the rate at whichit charges.To ensure a
valid reset the RST pin must be held high long enough
to allow the oscillator to stsrt up plus two machine
cycles.
On power up, VCCshould rise within approximately
ten milliseconds.The oscillator start-up time will de-
pendon the oscillatorfrequency.Fora 10MHz crystal,
the start-up timeis typically 1rns.For a 1MHz crystal,
the start-up time is typically 10ms.
With the givencircui~ reducingVW quicklyto Ocaus-
es the RST pin voltageto momentarilyfall below OV.
However,this voltageis internzdlylimitedand will not
harm the device.
NOTE:
The port pins will be in a random state until
the oscillatorhas started and the internal reset
algorithmhas written 1s to them.
Powering up the device without a valid reset could
cause the CPU to start executinginstructionsfrom an
indeterrninatelocation. This is becausethe SFRs, ape-
citically the Program Counter, may not get properly
initialized.
POWER-SAVING MODESOF
OPERATION
For applicationswhere power consumptionis critical
the CHMOSversionprovideapowerreducedmodesof
operationas a standard feature. The powerdownmode
in HMOS
devices i s
no
longer a standard featureandis
being
phased
OUt.
CHMOSPowerReductionModes
CHMOS versions have two power-reducingmodes,
Idle and PowerDown.The input throughwhichback-
up power is suppliedduring these operationsis VCC.
Figure 27 shows the internal circuitry which imple-
ments these features. In the Idle mode(IDL = 1), the
oscillator continuea to run and the Interrupt, Serial
Port, and Timer blockscontinueto be clocked,but the
3-27