
SMSC DS – FDC37N958FR
Page 221
Rev. 09/01/99
Power Management
The HOSTD signal controls all host bus inputs to
the RTC and RAM (nIOW, nIOR, VCC1 POR).
When asserted, it disallows any modification of
the RTC and RAM data by the host. HOSTD is
asserted whenever:
1. V
cc
2 is below 4.0 volts nominal
2. PowerGood is inactive and V
cc
2 is above
4.0 volts nominal
The 8051D signal controls all 8051 inputs to the
RTC and RAM. When asserted, it disallows any
modification of the RTC and RAM data by the
8051. 8051D is asserted whenever:
1. V
cc
1 is below 2.5 volts nominal.
2. V
cc
1 is above 2.5 volts and the 8051 is in its
hardware initialization routine.
The RTC (and CMOS) always draws power from
VCC0.
When the V
cc
2 voltage drops below 4.0 volts
nominal, all host inputs are locked out so that
the internal registers cannot be modified by the
host system. This lockout condition continues
for 500usec (min) to 1msec (max) after the
VCC2 power has been restored. The timed
lockout does not occur under the following
conditions:
1. The Divider Chain Controls (bits 6-4) are in
any mode but Normal Operation ("010").
2. The VRT bit is a "0".
To minimize power consumption, the oscillator is
not operational under the following conditions:
1. The Divider Chain Controls (bits 6-4) are in
Oscillator Disabled mode (000, or 001).
2. If VCC1=0 and the VCC0 is removed and
the re-applied (a new battery is installed) the
following occurs:
a) The oscillator is disabled immediately.
b) Initialize all registers 00-0D to a “00”
when VCC1 is applied.
Note:
system VCC0, VCC1 and VCC2. VCC0 must
be present before or at the same time as VCC1.
There are three power supplies in the
VCC1 must be present before or at the same
time as VCC2. The RTC and CMOS registers
always draw power from VCC0.
VCC2
(NOMINAL)
<4.0
<4.0 to >4.0
>4.0
POWER
GOOD
x
0
0->1
BATTERY VOLTAGE
>2.5V
Y
Y
Y
HOST REGISTER
ACCESS
N
N
Timed Lockout
(Note 1)
N
Y
>4.0
>4.0
0
1
Y
Y
Note 1: If VCC2 and VCC1 are powered up at the same time, then the Host Register
Access is delayed by the timed lockout and the 8051 Initialization, whichever is longer.
VCC1
(NOMINAL)
<2.5
<2.5 to >2.5
>2.5
>2.5
VCC2
(NOMINAL)
x
x
x
x
8051
INITIALIZATION
x
In Init
In Init
Init Finished
8051 REGISTER
ACCESS
N
N
N
Y