Micronas SDA 6000, Руководство пользователя

Страница: 129 / 412

SDA 6000
PRELIMINARY DATA SHEET
Version 2.1
System Control & Configuration
6 - 9 Micronas
6.3 Register Write Protection
The System Control Unit (SCU) provides two different protection types of
configuration registers:
• Unprotected Registers
• Protectable Registers
The unprotected registers allow the reading and writing (if not read-only) of register
values without any restrictions. However, the write access of the protectable registers
(security registers) can be programmed for
three different modes of security level ,
whereas the read access is always unprotected:
• Write Protected Mode
• Low Protected Mode
• Unprotected Mode
In write protected mode, the registers can not be accessed by a write command.
However, in low protected mode, the registers can be written with a special command
sequence (see description below). If the registers are set to unprotected mode, all write
accesses are possible.
Some register controlled functions and modes which are critical for M2’s operation are
locked after the execution of EINIT, so these vital system functions cannot be changed
inadvertently e.g. by software errors. However, as these security registers also control
the power management, they need to be accessed during operation to select the
appropriate mode.
The switching between the different security levels is controlled by a state machine. By
using a password and command sequence the security levels can be changed. After
reset the unprotected mode is always automatically selected. The EINIT command
switches the security level automatically to protected mode.
The low protected mode is especially important for a standby state of the application.
This mode allows fast accesses within 2 commands to the protected registers without
removing the protection completely.
Security Level Switching
Two registers are provided for switching the security level, the security level command
register SCUSLC and the security level status register SCUSLS. The security level
command register SCUSLC is used to control the state machine for switching the
security level. The SCUSLC register is loaded with the different commands of the
command sequence necessary to control a change in the security level. It is also used
for the one unlock command, which is necessary in the low protected mode to access
one protected register. The commands of the (unlock) command sequence are
characterized by certain pattern words (such as AAAA
H
) or by patterns combined with
an 8-bit password. For command definition see the following state diagram ( Figure 6-1 ).