Техническая Спецификация для Atmel Xplained Pro Evaluation Kit ATSAMD20-XPRO ATSAMD20-XPRO

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Atmel | SMART SAM D20 [DATASHEET]

Atmel-42129K–SAM-D20_datasheet–06/2014

The AHB-APB bridge is an AHB slave, providing an interface between the high-speed AHB domain and the low-power 
APB domain. It is used to provide access to the programmable control registers of peripherals (see 

).

to operate the AHB-APB bridge, the clock (CLK_HPBx_AHB) must be enabled. See 

 

for details.

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.

The PAC peripheral bus clock (CLK_PACx_APB) is enabled by default, and can be enabled and disabled in the Power 
Manager. 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.

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.

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).

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.