Production Data
WM8321
20.4 DIGITAL RIGHTS MANAGEMENT
The Real Time Clock (RTC) maintains a continuous record of the time; this is maintained at all times,
including when the WM8321 is powered down and the RTC function is maintained by the backup
battery.
It is highly desirable to be able to write to the RTC counter in order to configure it for logical
translation into hours/minutes and to support calendar functions. However, for digital rights
management purposes, it is important that malicious modification of the RTC is either prevented or
detected.
The security measure implemented on the WM8321 is the RTC Write Counter. This register is
initialised to 0000h during Power On Reset, and is updated automatically whenever a Write operation
is scheduled on either of the RTC_TIME registers. Note that, when the RTC Write Counter is updated,
the new value is generated at random; it is not a sequential counter.
It is assumed that legitimate updates to the RTC_TIME are only those initiated by the Application
Processor (AP). When the AP makes an update to the RTC_TIME, the AP can also read the new
value of the RTC Write Counter, and should store the value in non-volatile memory. If the AP detects
a change in value of the RTC Write Counter, and this was not caused by the AP itself writing to the
RTC_TIME, this means that an unauthorised write to the RTC_TIME registers has occurred.
In order to make it difficult for an unauthorised RTC_TIME update to be masked by simply writing to
the RTC Write Counter, the RTC_WR_CNT field is generated at random by the WM8321 whenever
the RTC_TIME field is updated.
For additional security, the WM8321 does not allow the RTC to be updated more than 8 times in a
one-hour period. Additional write attempts will be ignored.
The RTC Control registers are described in Table 49.
20.5 BACKUP MODE CLOCKING OPTIONS
The BACKUP state is entered when the PVDD power supply is below the reset threshold of the
device. Typically, this means that the PVDD supply has been removed. Most of the device functions
and registers are reset in this state.
The RTC and oscillator and a ‘software scratch’ memory area can be maintained from a backup
power source in the BACKUP state. This is provided using a coin cell, super/gold capacitor, or else a
standard capacitor, connected to the LDO12VOUT pin via a 22kꢀ resistor. See Section 17.3 for
further details.
The RTC and oscillator can be disabled in the BACKUP state by setting the XTAL_BKUPENA register
bit to 0. This feature may be used to minimise the device power consumption in the BACKUP state. A
22F capacitor connected to LDO12VOUT can maintain the RTC value, unclocked, for up to 5
minutes in BACKUP if the oscillator is disabled.
The XTAL_BKUPENA register bit is defined in Section 13.1. For more details on backup power, see
Section 17.3.
PD, February 2012, Rev 4.0
107
w
WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
