SMART ARM-based Microcontrollers
AT03229: SAM D/R/L/C Peripheral Access
Controller (PAC) Driver
APPLICATION NOTE
Introduction
®
®
This driver for Atmel | SMART ARM -based microcontroller provides an
interface for the locking and unlocking of peripheral registers within the
device. When a peripheral is locked, accidental writes to the peripheral will
be blocked and a CPU exception will be raised.
The following peripherals are used by this module:
•
PAC (Peripheral Access Controller)
The following devices can use this module:
•
Atmel | SMART SAM D20/D21
•
Atmel | SMART SAM R21
•
Atmel | SMART SAM D09/D10/D11
•
Atmel | SMART SAM L21/L22
•
Atmel | SMART SAM DA1
•
Atmel | SMART SAM C20/C21
The outline of this documentation is as follows:
•
Prerequisites
•
Module Overview
•
Special Considerations
•
Extra Information
•
Examples
•
API Overview
Atmel-42107F-SAM-Peripheral-Access-Controller-PAC-Driver_AT03229_Application Note-12/2015
Table of Contents
Introduction......................................................................................................................1
1. Software License....................................................................................................... 3
2. Prerequisites..............................................................................................................4
3. Module Overview....................................................................................................... 5
3.1.
3.2.
3.3.
3.4.
3.5.
3.6.
3.7.
Locking Scheme...........................................................................................................................5
Recommended Implementation....................................................................................................5
Why Disable Interrupts................................................................................................................. 7
Run-away Code............................................................................................................................8
3.4.1.
Key-Argument..............................................................................................................10
Faulty Module Pointer.................................................................................................................11
Use of __no_inline...................................................................................................................... 11
Physical Connection................................................................................................................... 11
4. Special Considerations............................................................................................ 12
4.1.
4.2.
Non-Writable Registers.............................................................................................................. 12
Reading Lock State.................................................................................................................... 12
5. Extra Information..................................................................................................... 13
6. Examples................................................................................................................. 14
7. API Overview........................................................................................................... 15
7.1.
7.2.
Macro Definitions........................................................................................................................15
7.1.1.
Macro SYSTEM_PERIPHERAL_ID.............................................................................15
Function Definitions....................................................................................................................15
7.2.1.
Peripheral Lock and Unlock.........................................................................................15
7.2.2.
APIs available for SAM L21/L22/C20/C21...................................................................16
8. Extra Information for PAC Driver............................................................................. 18
8.1.
8.2.
8.3.
8.4.
Acronyms....................................................................................................................................18
Dependencies.............................................................................................................................18
Errata..........................................................................................................................................18
Module History............................................................................................................................18
9. Examples for PAC Driver......................................................................................... 20
9.1.
Quick Start Guide for PAC - Basic..............................................................................................20
9.1.1.
Setup........................................................................................................................... 20
9.1.2.
Use Case..................................................................................................................... 20
10. List of Non-Write Protected Registers..................................................................... 23
11. Document Revision History..................................................................................... 25
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1.
Software License
Redistribution and use in source and binary forms, with or without modification, are permitted provided
that the following conditions are met:
1. Redistributions of source code must retain the above copyright notice, this list of conditions and the
following disclaimer.
2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the
following disclaimer in the documentation and/or other materials provided with the distribution.
3. The name of Atmel may not be used to endorse or promote products derived from this software without
specific prior written permission.
4. This software may only be redistributed and used in connection with an Atmel microcontroller product.
THIS SOFTWARE IS PROVIDED BY ATMEL "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS
FOR A PARTICULAR PURPOSE AND NON-INFRINGEMENT ARE EXPRESSLY AND SPECIFICALLY
DISCLAIMED. IN NO EVENT SHALL ATMEL BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER
IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
SUCH DAMAGE.
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2.
Prerequisites
There are no prerequisites for this module.
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3.
Module Overview
The SAM devices are fitted with a Peripheral Access Controller (PAC) that can be used to lock and unlock
write access to a peripheral's registers (see Non-Writable Registers). Locking a peripheral minimizes the
risk of unintended configuration changes to a peripheral as a consequence of Run-away Code or use of a
Faulty Module Pointer.
Physically, the PAC restricts write access through the AHB bus to registers used by the peripheral,
making the register non-writable. PAC locking of modules should be implemented in configuration critical
applications where avoiding unintended peripheral configuration changes are to be regarded in the
highest of priorities.
All interrupt must be disabled while a peripheral is unlocked to make sure correct lock/unlock scheme is
upheld.
3.1.
Locking Scheme
The module has a built in safety feature requiring that an already locked peripheral is not relocked, and
that already unlocked peripherals are not unlocked again. Attempting to unlock and already unlocked
peripheral, or attempting to lock a peripheral that is currently locked will generate a CPU exception. This
implies that the implementer must keep strict control over the peripheral's lock-state before modifying
them. With this added safety, the probability of stopping runaway code increases as the program pointer
can be caught inside the exception handler, and necessary countermeasures can be initiated. The
implementer should also consider using sanity checks after an unlock has been performed to further
increase the security.
3.2.
Recommended Implementation
A recommended implementation of the PAC can be seen in Figure 3-1 Recommended Implementation
on page 6.
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Figure 3-1. Recommended Implementation
Initialization and code
Initialize Peripheral
Lock peripheral
Other initialization
and enable interrupts if applicable
Peripheral Modification
Disable global interrupts
Unlock peripheral
Sanity Check
Modify peripheral
Lock peripheral
Enable global interrupts
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3.3.
Why Disable Interrupts
Global interrupts must be disabled while a peripheral is unlocked as an interrupt handler would not know
the current state of the peripheral lock. If the interrupt tries to alter the lock state, it can cause an
exception as it potentially tries to unlock an already unlocked peripheral. Reading current lock state is to
be avoided as it removes the security provided by the PAC (Reading Lock State).
Note: Global interrupts should also be disabled when a peripheral is unlocked inside an interrupt
handler.
An example to illustrate the potential hazard of not disabling interrupts is shown in Figure 3-2 Why
Disable Interrupts on page 8.
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Figure 3-2. Why Disable Interrupts
Main routine
Initialize and lock peripherals
User code
Unlock peripheral
Modify peripheral
Interrupt
Interrupt handler
Lock peripheral
Unlock peripheral
Modify peripheral
Exception
Lock peripheral
3.4.
Run-away Code
Run-away code can be caused by the MCU being operated outside its specification, faulty code, or EMI
issues. If a runaway code occurs, it is favorable to catch the issue as soon as possible. With a correct
implementation of the PAC, the runaway code can potentially be stopped.
A graphical example showing how a PAC implementation will behave for different circumstances of
runaway code in shown in Figure 3-3 Run-away Code on page 9 and Figure 3-4 Run-away Code on
page 10.
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Figure 3-3. Run-away Code
1. Run-away code is caught in sanity check.
A CPU exception is executed.
Run-away code
2. Run-away code is caught when modifying
locked peripheral. A CPU exception is executed.
Run-away code
PC#
Code
PC#
Code
0x0020
initialize peripheral
0x0020
initialize peripheral
0x0025
lock peripheral
0x0025
lock peripheral
...
...
...
...
0x0080
set sanity argument
0x0080
set sanity argument
...
...
...
...
0x0115
disable interrupts
0x0115
disable interrupts
0x0120
unlock peripheral
0x0120
unlock peripheral
0x0125 check sanity argument
0x0125 check sanity argument
0x0130
modify peripheral
0x0130
modify peripheral
0x0140
lock peripheral
0x0140
lock peripheral
0x0145
disable interrupts
0x0145
disable interrupts
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Figure 3-4. Run-away Code
3. Run-away code is caught when locking
locked peripheral. A CPU exception is executed.
Run-away code
4. Run-away code is not caught.
Run-away code
PC#
Code
PC#
Code
0x0020
initialize peripheral
0x0020
initialize peripheral
0x0025
lock peripheral
0x0025
lock peripheral
...
...
...
...
0x0080
set sanity argument
0x0080
set sanity argument
...
...
...
...
0x0115
disable interrupts
0x0115
disable interrupts
0x0120
unlock peripheral
0x0120
unlock peripheral
0x0125 check sanity argument
0x0125 check sanity argument
0x0130
modify peripheral
0x0130
modify peripheral
0x0140
lock peripheral
0x0140
lock peripheral
0x0145
disable interrupts
0x0145
disable interrupts
In the example, green indicates that the command is allowed, red indicates where the runaway code will
be caught, and the arrow where the runaway code enters the application. In special circumstances, like
example 4 above, the runaway code will not be caught. However, the protection scheme will greatly
enhance peripheral configuration security from being affected by runaway code.
3.4.1.
Key-Argument
To protect the module functions against runaway code themselves, a key is required as one of the input
arguments. The key-argument will make sure that runaway code entering the function without a function
call will be rejected before inflicting any damage. The argument is simply set to be the bitwise inverse of
the module flag, i.e.
system_peripheral_<lock_state>(SYSTEM_PERIPHERAL_<module>,
~SYSTEM_PERIPHERAL_<module>);
Where the lock state can be either lock or unlock, and module refer to the peripheral that is to be locked/
unlocked.
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3.5.
Faulty Module Pointer
The PAC also protects the application from user errors such as the use of incorrect module pointers in
function arguments, given that the module is locked. It is therefore recommended that any unused
peripheral is locked during application initialization.
3.6.
Use of __no_inline
Using the function attribute __no_inline will ensure that there will only be one copy of each functions in
the PAC driver API in the application. This will lower the likelihood that runaway code will hit any of these
functions.
3.7.
Physical Connection
Figure 3-5 Physical Connection on page 11 shows how this module is interconnected within the device.
Figure 3-5. Physical Connection
PAC
Read/Write
Peripheral bus
Read/Write
Lock
Open
Read
Read/Write
Peripheral1
Peripheral2
Read/Write
Open
Read/Write
Peripheral3
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4.
Special Considerations
4.1.
Non-Writable Registers
Not all registers in a given peripheral can be set non-writable. Which registers this applies to is showed in
List of Non-Write Protected Registers and the peripheral's subsection "Register Access Protection" in the
device datasheet.
4.2.
Reading Lock State
Reading the state of the peripheral lock is to be avoided as it greatly compromises the protection initially
provided by the PAC. If a lock/unlock is implemented conditionally, there is a risk that eventual errors are
not caught in the protection scheme. Examples indicating the issue are shown in Figure 4-1 Reading
Lock State on page 12.
Figure 4-1. Reading Lock State
1. Wrong implementation.
Run-away code
with peripheral unlocked
Run-away code
with peripheral unlocked
PC#
Code
...
...
0x0100
check if locked
0x0102
disable interrupts
0x0105
unlock if locked
0x0110
check sanity
0x0115
modify peripheral
0x0120 lock if previously locked
0x0125
2. Correct implementation.
enable interrupts
PC#
Code
...
...
0x0100
disable interrupts
0x0120
unlock peripheral
0x0125 check sanity argument
0x0130
modify peripheral
0x0140
lock peripheral
0x0145
disable interrupts
In the left figure above, one can see the runaway code continues as all illegal operations are conditional.
On the right side figure, the runaway code is caught as it tries to unlock the peripheral.
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5.
Extra Information
For extra information, see Extra Information for PAC Driver. This includes:
•
Acronyms
•
Dependencies
•
Errata
•
Module History
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6.
Examples
For a list of examples related to this driver, see Examples for PAC Driver.
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7.
API Overview
7.1.
Macro Definitions
7.1.1.
Macro SYSTEM_PERIPHERAL_ID
#define SYSTEM_PERIPHERAL_ID(peripheral)
Retrieves the ID of a specified peripheral name, giving its peripheral bus location.
Table 7-1. Parameters
Data direction
Parameter name
Description
[in]
peripheral
Name of the peripheral instance
7.2.
Function Definitions
7.2.1.
Peripheral Lock and Unlock
7.2.1.1.
Function system_peripheral_lock()
Lock a given peripheral's control registers.
__no_inline enum status_code system_peripheral_lock(
const uint32_t peripheral_id,
const uint32_t key)
Locks a given peripheral's control registers, to deny write access to the peripheral to prevent accidental
changes to the module's configuration.
Warning Locking an already locked peripheral will cause a CPU exception, and terminate program
execution.
Table 7-2. Parameters
Data direction Parameter name Description
[in]
peripheral_id
ID for the peripheral to be locked, sourced via the
SYSTEM_PERIPHERAL_ID macro
[in]
key
Bitwise inverse of peripheral ID, used as key to reduce the chance
of accidental locking. See Key-Argument
Returns
Status of the peripheral lock procedure.
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Table 7-3. Return Values
7.2.1.2.
Return value
Description
STATUS_OK
If the peripheral was successfully locked
STATUS_ERR_INVALID_ARG
If invalid argument(s) were supplied
Function system_peripheral_unlock()
Unlock a given peripheral's control registers.
__no_inline enum status_code system_peripheral_unlock(
const uint32_t peripheral_id,
const uint32_t key)
Unlocks a given peripheral's control registers, allowing write access to the peripheral so that changes can
be made to the module's configuration.
Warning Unlocking an already locked peripheral will cause a CUP exception, and terminate program
execution.
Table 7-4. Parameters
Data direction Parameter name Description
[in]
peripheral_id
ID for the peripheral to be unlocked, sourced via the
SYSTEM_PERIPHERAL_ID macro
[in]
key
Bitwise inverse of peripheral ID, used as key to reduce the chance
of accidental unlocking. See Key-Argument
Returns
Status of the peripheral unlock procedure.
Table 7-5. Return Values
Return value
Description
STATUS_OK
If the peripheral was successfully locked
STATUS_ERR_INVALID_ARG
If invalid argument(s) were supplied
7.2.2.
APIs available for SAM L21/L22/C20/C21.
7.2.2.1.
Function system_peripheral_lock_always()
Lock a given peripheral's control registers until hardware reset.
__no_inline enum status_code system_peripheral_lock_always(
const uint32_t peripheral_id,
const uint32_t key)
Locks a given peripheral's control registers, to deny write access to the peripheral to prevent accidental
changes to the module's configuration. After lock, the only way to unlock is hardware reset.
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Warning Locking an already locked peripheral will cause a CPU exception, and terminate program
execution.
Table 7-6. Parameters
Data direction Parameter name Description
[in]
peripheral_id
ID for the peripheral to be locked, sourced via the
SYSTEM_PERIPHERAL_ID macro
[in]
key
Bitwise inverse of peripheral ID, used as key to reduce the chance
of accidental locking. See Key-Argument
Returns
Status of the peripheral lock procedure.
Table 7-7. Return Values
7.2.2.2.
Return value
Description
STATUS_OK
If the peripheral was successfully locked
STATUS_ERR_INVALID_ARG
If invalid argument(s) were supplied
Function system_pac_enable_interrupt()
Enable PAC interrupt.
void system_pac_enable_interrupt( void )
Enable PAC interrupt so can trigger execution on peripheral access error, see SYSTEM_Handler().
7.2.2.3.
Function system_pac_disable_interrupt()
Disable PAC interrupt.
void system_pac_disable_interrupt( void )
Disable PAC interrupt on peripheral access error.
7.2.2.4.
Function system_pac_enable_event()
Enable PAC event output.
void system_pac_enable_event( void )
Enable PAC event output on peripheral access error.
7.2.2.5.
Function system_pac_disable_event()
Disable PAC event output.
void system_pac_disable_event( void )
Disable PAC event output on peripheral access error.
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8.
Extra Information for PAC Driver
8.1.
Acronyms
Below is a table listing the acronyms used in this module, along with their intended meanings.
8.2.
Acronym
Description
AC
Analog Comparator
ADC
Analog-to-Digital Converter
EVSYS
Event System
NMI
Non-Maskable Interrupt
NVMCTRL
Non-Volatile Memory Controller
PAC
Peripheral Access Controller
PM
Power Manager
RTC
Real-Time Counter
SERCOM
Serial Communication Interface
SYSCTRL
System Controller
TC
Timer/Counter
WDT
Watch Dog Timer
Dependencies
This driver has the following dependencies:
•
8.3.
None
Errata
There are no errata related to this driver.
8.4.
Module History
An overview of the module history is presented in the table below, with details on the enhancements and
fixes made to the module since its first release. The current version of this corresponds to the newest
version in the table.
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Changelog
Initial Release
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9.
Examples for PAC Driver
This is a list of the available Quick Start guides (QSGs) and example applications for SAM Peripheral
Access Controller (PAC) Driver. QSGs are simple examples with step-by-step instructions to configure
and use this driver in a selection of use cases. Note that a QSG can be compiled as a standalone
application or be added to the user application.
•
9.1.
Quick Start Guide for PAC - Basic
Quick Start Guide for PAC - Basic
In this use case, the peripheral-lock will be used to lock and unlock the PORT peripheral access, and
show how to implement the PAC module when the PORT registers needs to be altered. The PORT will be
set up as follows:
•
One pin in input mode, with pull-up and falling edge-detect
•
One pin in output mode
9.1.1.
Setup
9.1.1.1.
Prerequisites
There are no special setup requirements for this use-case.
9.1.1.2.
Code
Copy-paste the following setup code to your user application:
void config_port_pins(void)
{
struct port_config pin_conf;
port_get_config_defaults(&pin_conf);
pin_conf.direction = PORT_PIN_DIR_INPUT;
pin_conf.input_pull = PORT_PIN_PULL_UP;
port_pin_set_config(BUTTON_0_PIN, &pin_conf);
}
pin_conf.direction = PORT_PIN_DIR_OUTPUT;
port_pin_set_config(LED_0_PIN, &pin_conf);
Add to user application initialization (typically the start of main()):
config_port_pins();
9.1.2.
Use Case
9.1.2.1.
Code
Copy-paste the following code to your user application:
system_init();
config_port_pins();
system_peripheral_lock(SYSTEM_PERIPHERAL_ID(PORT),
~SYSTEM_PERIPHERAL_ID(PORT));
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#if(SAML21)|| (SAML22) || (SAMC21) || defined(__DOXYGEN__)
system_pac_enable_interrupt();
#endif
system_interrupt_enable_global();
while (port_pin_get_input_level(BUTTON_0_PIN)) {
/* Wait for button press */
}
system_interrupt_enter_critical_section();
system_peripheral_unlock(SYSTEM_PERIPHERAL_ID(PORT),
~SYSTEM_PERIPHERAL_ID(PORT));
port_pin_toggle_output_level(LED_0_PIN);
system_peripheral_lock(SYSTEM_PERIPHERAL_ID(PORT),
~SYSTEM_PERIPHERAL_ID(PORT));
system_interrupt_leave_critical_section();
while (1) {
/* Do nothing */
}
9.1.2.2.
Workflow
1.
Configure some GPIO port pins for input and output.
config_port_pins();
2.
Lock peripheral access for the PORT module; attempting to update the module while it is in a
protected state will cause a CPU exception. For SAM D20/D21/D10/D11/R21/DA0/DA1, it is Hard
Fault exception; For SAM L21/C21, it is system exception, see SYSTEM_Handler().
system_peripheral_lock(SYSTEM_PERIPHERAL_ID(PORT),
~SYSTEM_PERIPHERAL_ID(PORT));
3.
Enable global interrupts.
#if (SAML21) || (SAML22) || (SAMC21) || defined(__DOXYGEN__)
system_pac_enable_interrupt();
#endif
system_interrupt_enable_global();
4.
Loop to wait for a button press before continuing.
while (port_pin_get_input_level(BUTTON_0_PIN)) {
/* Wait for button press */
}
5.
Enter a critical section, so that the PAC module can be unlocked safely and the peripheral
manipulated without the possibility of an interrupt modifying the protected module's state.
system_interrupt_enter_critical_section();
6.
Unlock the PORT peripheral registers.
system_peripheral_unlock(SYSTEM_PERIPHERAL_ID(PORT),
~SYSTEM_PERIPHERAL_ID(PORT));
7.
Toggle pin 11, and clear edge detect flag.
port_pin_toggle_output_level(LED_0_PIN);
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8.
Lock the PORT peripheral registers.
system_peripheral_lock(SYSTEM_PERIPHERAL_ID(PORT),
~SYSTEM_PERIPHERAL_ID(PORT));
9.
Exit the critical section to allow interrupts to function normally again.
system_interrupt_leave_critical_section();
10. Enter an infinite while loop once the module state has been modified successfully.
while (1) {
/* Do nothing */
}
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10.
List of Non-Write Protected Registers
Look in device datasheet peripheral's subsection "Register Access Protection" to see which is actually
available for your device.
Module
Non-write protected register
AC
INTFLAG
STATUSA
STATUSB
STATUSC
ADC
INTFLAG
STATUS
RESULT
EVSYS
INTFLAG
CHSTATUS
NVMCTRL
INTFLAG
STATUS
PM
INTFLAG
PORT
N/A
RTC
INTFLAG
READREQ
STATUS
SYSCTRL
INTFLAG
SERCOM
INTFALG
STATUS
DATA
TC
INTFLAG
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Module
Non-write protected register
STATUS
WDT
INTFLAG
STATUS
(CLEAR)
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11.
Document Revision History
Doc. Rev.
Date
Comments
42107F
12/2015
Added support for SAM L21/L22, SAM C20/C21, SAM D09, and SAM DA1
42107E
12/2014
Added support for SAM R21 and SAM D10/D11
42107D
01/2014
Added support for SAM D21
42107C
10/2013
Extended acronyms list
42107B
06/2013
Corrected documentation typos
42107A
06/2013
Initial document release
Atmel AT03229: SAM D/R/L/C Peripheral Access Controller (PAC) Driver [APPLICATION NOTE]
Atmel-42107F-SAM-Peripheral-Access-Controller-PAC-Driver_AT03229_Application Note-12/2015
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2015 Atmel Corporation. / Rev.: Atmel-42107F-SAM-Peripheral-Access-Controller-PAC-Driver_AT03229_Application Note-12/2015
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