4. The Auto powerdown timer (10msec) must
have timed out.
Register Behavior
TABLE 48 illustrates the AT and PS/2 (including
Model 30) configuration registers available and the
type of access permitted. In order to maintain
software transparency, access to all the registers
must be maintained. As TABLE 48 shows, two
sets of registers are distinguished based on
whether their access results in the part remaining
in powerdown state or exiting it.
An internal timer is initiated as soon as the auto
powerdown command is enabled. The part is then
powered down when all the conditions are met.
Disabling the auto powerdown mode cancels the
timer and holds the FDC block out of auto
powerdown.
DSR From Powerdown
Access to all other registers is possible without
awakening the part. These registers can be
accessed during powerdown without changing the
status of the part. A read from these registers will
reflect the true status as shown in the register
description in the FDC description. A write to the
part will result in the part retaining the data and
subsequently reflecting it when the part awakens.
Accessing the part during powerdown may cause
an increase in the power consumption by the part.
The part will revert back to its low power mode
when the access has been completed.
If DSR powerdown is used when the part is in auto
powerdown, the DSR powerdown will override the
auto powerdown. However, when the part is
awakened from DSR powerdown, the auto
powerdown will once again become effective.
Wake Up From Auto Powerdown
If the part enters the powerdown state through the
auto powerdown mode, then the part can be
awakened by reset or by appropriate access to
certain registers.
Pin Behavior
If a hardware or software reset is used then the
part will go through the normal reset sequence. If
the access is through the selected registers, then
the FDC resumes operation as though it was
This chip is specifically designed for systems in
which power conservation is a primary concern.
This makes the behavior of the pins during
powerdown very important.
never in powerdown.
Besides activating the
RESET pin or one of the software reset bits in the
DOR or DSR, the following register accesses will
wake up the part:
The pins can be divided into two major categories:
system interface and floppy disk drive interface.
The floppy disk drive pins are disabled so that no
power will be drawn through the part as a result of
any voltage applied to the pin within the part's
power supply range. Most of the system interface
pins are left active to monitor system accesses
that may wake up the part.
1. Enabling any one of the motor enable bits in
the DOR register (reading the DOR does not
awaken the part).
2. A read from the MSR register.
System Interface Pins
3. A read or write to the Data register.
TABLE 49 gives the state of the system interface
pins in the powerdown state. Pins unaffected by
the powerdown are labeled "Unchanged". Input
pins are "Disabled" to prevent them from causing
currents internal to the chip when they have
indeterminate input values.
Once awake, the FDC will reinitiate the auto
powerdown timer for 10 ms. The part will
powerdown again when all the powerdown
conditions are satisfied.
112
SMSC [ SMSC CORPORATION ]
