Atmel Xplained Pro Evaluation Kit for the ATSAMD21J18A Microcontroller ATSAMD21-XPRO ATSAMD21-XPRO Data Sheet
Product codes
ATSAMD21-XPRO
33
Atmel | SMART SAM D21 [DATASHEET]
Atmel-42181C–SAM-D21_Datasheet–07/2014
Figure 10-2. APB Read Access
.
10.6 PAC – Peripheral Access Controller
10.6.1 Overview
There is one PAC associated with each AHB-APB bridge. The PAC can provide write protection for registers of each
peripheral connected on the same bridge.
peripheral connected on the same bridge.
The PAC peripheral bus clock (CLK_PACx_APB) can be enabled and disabled in the Power Manager. CLK_PAC0_APB
and CLK_PAC1_APB are enabled are reset. CLK_PAC2_APB is disabled at reset. Refer to
and CLK_PAC1_APB are enabled are reset. CLK_PAC2_APB is disabled at reset. Refer to
for details. The PAC will continue to operate in any sleep mode where the selected clock source is running.
Write-protection does not apply for debugger access. When the debugger makes an access to a peripheral, write-
protection is ignored so that the debugger can update the register.
protection is ignored so that the debugger can update the register.
Write-protect registers allow the user to disable a selected peripheral’s write-protection without doing a read-modify-write
operation. These registers are mapped into two I/O memory locations, one for clearing and one for setting the register
bits. Writing a one to a bit in the Write Protect Clear register (WPCLR) will clear the corresponding bit in both registers
(WPCLR and WPSET) and disable the write-protection for the corresponding peripheral, while writing a one to a bit in the
Write Protect Set (WPSET) register will set the corresponding bit in both registers (WPCLR and WPSET) and enable the
write-protection for the corresponding peripheral. Both registers (WPCLR and WPSET) will return the same value when
read.
operation. These registers are mapped into two I/O memory locations, one for clearing and one for setting the register
bits. Writing a one to a bit in the Write Protect Clear register (WPCLR) will clear the corresponding bit in both registers
(WPCLR and WPSET) and disable the write-protection for the corresponding peripheral, while writing a one to a bit in the
Write Protect Set (WPSET) register will set the corresponding bit in both registers (WPCLR and WPSET) and enable the
write-protection for the corresponding peripheral. Both registers (WPCLR and WPSET) will return the same value when
read.
If a peripheral is write-protected, and if a write access is performed, data will not be written, and the peripheral will return
an access error (CPU exception).
an access error (CPU exception).
The PAC also offers a safety feature for correct program execution, with a CPU exception generated on double write-
protection or double unprotection of a peripheral. If a peripheral n is write-protected and a write to one in WPSET[n] is
detected, the PAC returns an error. This can be used to ensure that the application follows the intended program flow by
always following a write-protect with an unprotect, and vice versa. However, in applications where a write-protected
peripheral is used in several contexts, e.g., interrupts, care should be taken so that either the interrupt can not happen
while the main application or other interrupt levels manipulate the write-protection status, or when the interrupt handler
needs to unprotect the peripheral, based on the current protection status, by reading WPSET.
protection or double unprotection of a peripheral. If a peripheral n is write-protected and a write to one in WPSET[n] is
detected, the PAC returns an error. This can be used to ensure that the application follows the intended program flow by
always following a write-protect with an unprotect, and vice versa. However, in applications where a write-protected
peripheral is used in several contexts, e.g., interrupts, care should be taken so that either the interrupt can not happen
while the main application or other interrupt levels manipulate the write-protection status, or when the interrupt handler
needs to unprotect the peripheral, based on the current protection status, by reading WPSET.
T0
T1
T2
T3
Addr 1
Data 1
PADDR
PWRITE
PCLK
PSEL
PENABLE
PRDATA
PREADY
T0
T1
T2
T3
Addr 1
Data 1
PADDR
PWRITE
PCLK
PSEL
PENABLE
PRDATA
PREADY
T4
T5
Wait states
No wait states