CTRL-AP - Control access port

The control access port (CTRL-AP) is a custom access port that enables control of the device when other debug access ports (DAP) have been disabled by the access port protection.

For an overview of the other debug access ports, see DAP - Debug access port.

Figure 1. Control access port details

Access port protection (APPROTECT) blocks the debugger access to the AHB-AP, and prevents read and write access to all CPU registers and memory-mapped addresses. To enable port protection access for both secure and non-secure modes, use the registers SECUREAPPROTECT and APPROTECT respectively. The debugger can use the register APPROTECT.STATUS to read the status of secure and non-secure access port protection.

CTRL-AP has the following features:

Soft reset
Erase all
Mailbox interface
Debug of protected devices

Reset request

The debugger can request the device to perform a soft reset.

Use the register RESET to request a soft reset. Once the soft reset is performed, the reset reason is accessible on the on-chip firmware through the RESETREAS register. For more information about the soft reset, see Reset.

Erase all

The Erase all function lets the debugger trigger an erase of flash, user information configuration registers (UICR), RAM, all peripheral settings, and also removes the access port protection.

To trigger an erase all function, the debugger writes to the register ERASEALL. The register ERASEALLSTATUS will read as busy for the duration of the operation. After the next reset, the access port protection is removed.

If the debugger performs an erase all function on a slave MCU, the erase sequence will always erase the application MCU first, independently of how the application is protected, before erasing the slave MCU.

Erase all protection

It is possible to prevent the debugger from performing an erase all operation by writing to the UICR.ERASEPROTECT register. Once the register is configured and the device is reset, the CTRL-AP ERASEALL operation is disabled, and all flash write and erase operations are restricted to the firmware. In addition, it is still possible to write/erase from the debugger as long as the UICR.APPROTECT register is not set.

Note: Setting the UICR.ERASEPROTECT register only affects the erase all operation and not the debugger access.

The register ERASEPROTECT.STATUS holds the status for erase protection.

Mailbox interface

CTRL-AP implements a mailbox interface which enables the CPU to communicate with a debugger over the SWD interface.

The mailbox interface consists of a transmit register MAILBOX.TXDATA with its corresponding status register MAILBOX.TXSTATUS, and a receive register MAILBOX.RXDATA with its corresponding status register MAILBOX.RXSTATUS. Status bits in the TXSTATUS/RXSTATUS registers are set and cleared automatically when the TXDATA/RXDATA registers are written to and read from, independently of the direction.

Figure 2. Mailbox register interface

Mailbox register interface, illustrating the data flow between debugger and CPU

Mailbox transfer sequence

Sender writes TXDATA.
Hardware sets sender's TXSTATUS to DataPending.
Hardware sets receiver's RXSTATUS to DataPending.
Receiver reads RXDATA.
Hardware sets receiver's RXSTATUS to NoDataPending.
Hardware sets sender's TXSTATUS to NoDataPending.

Disabling erase protection

The erase protection mechanism can be disabled to return a device to factory default settings on next reset.

The debugger can read the erase protection status in the register ERASEPROTECT.STATUS.

If ERASEPROTECT has been enabled, both the debugger and on-chip firmware must write the same non-zero 32-bit KEY value into their respective ERASEPROTECT.DISABLE registers to disable the erase protection. When both registers have been written with the same non-zero 32-bit KEY value, the device is automatically erased as described in Erase all. The access ports will be re-enabled on the next reset once the secure erase sequence has completed.

The write-once register ERASEPROTECT.LOCK should be set to Locked as early as possible in the start-up sequence, preferably as soon as the on-chip firmware has determined it does not need to communicate with a debugger over the CTRL-AP mailbox interface. Once written, it will not be possible to remove the erase protection until the next reset.

Debugger registers

CTRL-AP has a set of registers that can only be accessed from the debugger over the SWD interface. These are not accessible from the CPU.

Registers

Table 1. Register overview
Register	Offset	Description
RESET	0x000	System reset request
ERASEALL	0x004	Perform a secure erase of the device, where flash, SRAM and UICR will be erased in sequence. The device will be returned to factory default settings upon next reset.
ERASEALLSTATUS	0x008	This is the status register for the ERASEALL operation.
APPROTECT.STATUS	0x00C	This is the status register for the UICR access port protection.
ERASEPROTECT.STATUS	0x018	This is the status register for the UICR ERASEPROTECT configuration.
ERASEPROTECT.DISABLE	0x01C	This register disables ERASEPROTECT and performs ERASEALL.
MAILBOX.TXDATA	0x020	Data sent from the debugger to the CPU.
MAILBOX.TXSTATUS	0x024	This register shows a status that indicates if data sent from the debugger to the CPU has been read.
MAILBOX.RXDATA	0x028	Data sent from the CPU to the debugger.
MAILBOX.RXSTATUS	0x02C	This register shows a status that indicates if data sent from the CPU to the debugger has been read.
IDR	0x0FC	CTRL-AP Identification Register, IDR

RESET

Address offset: 0x000

System reset request

This register is automatically deactivated by writing Erase to ERASEALL, it is then kept inactive until a reset source affecting the debug system is asserted. See Reset behavior.

Bit number				31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
ID																																			A
Reset 0x00000000				0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
ID	R/W	Field	Value ID	Value								Description
A	RW	RESET										System reset request and status
			NoReset	0								Write to release reset Reading '0' means reset is not active
			Reset	1								Write to hold reset Reading '1' means reset is active

ERASEALL

Address offset: 0x004

Perform a secure erase of the device, where flash, SRAM and UICR will be erased in sequence. The device will be returned to factory default settings upon next reset.

Bit number				31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
ID																																			A
Reset 0x00000000				0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
ID	R/W	Field	Value ID	Value								Description
A	W	ERASEALL										Return device to factory default settings
			NoOperation	0								No operation
			Erase	1								Erase flash, SRAM, and UICR in sequence

ERASEALLSTATUS

Address offset: 0x008

This is the status register for the ERASEALL operation.

Bit number				31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
ID																																			A
Reset 0x00000000				0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
ID	R/W	Field	Value ID	Value								Description
A	R	ERASEALLSTATUS										Status bit for the ERASEALL operation
			Ready	0								ERASEALL is ready
			Busy	1								ERASEALL is busy (on-going)

APPROTECT.STATUS

Address offset: 0x00C

This is the status register for the UICR access port protection.

Bit number				31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
ID																																		B	A
Reset 0x00000000				0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
ID	R/W	Field	Value ID	Value								Description
A	R	APPROTECT										Status bit for access port protection Note: The reset value is auto read from the APPROTECT register in UICR.
			Enabled	0								APPROTECT is enabled
			Disabled	1								APPROTECT is disabled
B	R	SECUREAPPROTECT										Status bit for secure access port protection Note: The reset value is auto read from the SECUREAPPROTECT register in UICR.
			Enabled	0								SECUREAPPROTECT is enabled
			Disabled	1								SECUREAPPROTECT is disabled

ERASEPROTECT.STATUS

Address offset: 0x018

This is the status register for the UICR ERASEPROTECT configuration.

Bit number				31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
ID																																			A
Reset 0x00000000				0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
ID	R/W	Field	Value ID	Value								Description
A	R	PALL										Status bit for erase protection Note: The reset value is auto read from the ERASEPROTECT register in UICR.
			Enabled	0								ERASEPROTECT is enabled
			Disabled	1								ERASEPROTECT is not enabled and ERASEALL can be performed

ERASEPROTECT.DISABLE

Address offset: 0x01C

This register disables ERASEPROTECT and performs ERASEALL.

Bit number				31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
ID				A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A
Reset 0x00000000				0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
ID	R/W	Field	Value ID	Value								Description
A	RW	KEY										The ERASEALL sequence will be initiated if value of the KEY fields are non-zero and the KEY fields match on both the CPU and debugger sides.

MAILBOX.TXDATA

Address offset: 0x020

Data sent from the debugger to the CPU.

Writing to this register will automatically set a DataPending value in the TXSTATUS register.

Bit number				31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
ID				A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A
Reset 0x00000000				0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
ID	R/W	Field	Value ID	Value								Description
A	RW	Data										Data sent from debugger

MAILBOX.TXSTATUS

Address offset: 0x024

This register shows a status that indicates if data sent from the debugger to the CPU has been read.

Bit number				31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
ID																																			A
Reset 0x00000000				0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
ID	R/W	Field	Value ID	Value								Description
A	R	Status										Status of register DATA
			NoDataPending	0								No data pending in register TXDATA
			DataPending	1								Data pending in register TXDATA

MAILBOX.RXDATA

Address offset: 0x028

Data sent from the CPU to the debugger.

Reading from this register will automatically set a NoDataPending value in the RXSTATUS register.

Bit number				31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
ID				A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A
Reset 0x00000000				0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
ID	R/W	Field	Value ID	Value								Description
A	R	Data										Data sent from CPU

MAILBOX.RXSTATUS

Address offset: 0x02C

This register shows a status that indicates if data sent from the CPU to the debugger has been read.

Bit number				31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
ID																																			A
Reset 0x00000000				0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
ID	R/W	Field	Value ID	Value								Description
A	R	Status										Status of register DATA
			NoDataPending	0								No data pending in register RXDATA
			DataPending	1								Data pending in register RXDATA

IDR

Address offset: 0x0FC

CTRL-AP Identification Register, IDR

Bit number				31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
ID				E	E	E	E	D	D	D	D	C	C	C	C	C	C	C	B	B	B	B						A	A	A	A	A	A	A	A
Reset 0x12880000				0	0	0	1	0	0	1	0	1	0	0	0	1	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
ID	R/W	Field	Value ID	Value								Description
A	R	APID										AP Identification
B	R	CLASS										Access Port (AP) class
			NotDefined	0x0								No defined class
			MEMAP	0x8								Memory Access Port
C	R	JEP106ID										JEDEC JEP106 identity code
D	R	JEP106CONT										JEDEC JEP106 continuation code
E	R	REVISION										Revision

Registers

Table 2. Instances
Base address	Peripheral	Instance	Secure mapping	DMA security	Description	Configuration
0x50006000	CTRLAPPERI	CTRL_AP_PERI	S	NA	CTRL-AP-PERI

Table 3. Register overview
Register	Offset	Description
MAILBOX.RXDATA	0x400	Data sent from the debugger to the CPU.
MAILBOX.RXSTATUS	0x404	This register shows a status that indicates if data sent from the debugger to the CPU has been read.
MAILBOX.TXDATA	0x480	Data sent from the CPU to the debugger.
MAILBOX.TXSTATUS	0x484	This register shows a status that indicates if the data sent from the CPU to the debugger has been read.
ERASEPROTECT.LOCK	0x500	This register locks the ERASEPROTECT.DISABLE register from being written until next reset.
ERASEPROTECT.DISABLE	0x504	This register disables the ERASEPROTECT register and performs an ERASEALL operation.

MAILBOX.RXDATA

Address offset: 0x400

Data sent from the debugger to the CPU.

Reading from this register will automatically set a NoDataPending value in the RXSTATUS register.

Bit number				31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
ID				A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A
Reset 0x00000000				0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
ID	R/W	Field	Value ID	Value								Description
A	R	RXDATA										Data received from debugger

MAILBOX.RXSTATUS

Address offset: 0x404

This register shows a status that indicates if data sent from the debugger to the CPU has been read.

Bit number				31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
ID																																			A
Reset 0x00000000				0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
ID	R/W	Field	Value ID	Value								Description
A	R	RXSTATUS										Status of data in register RXDATA
			NoDataPending	0								No data pending in register RXDATA
			DataPending	1								Data pending in register RXDATA

MAILBOX.TXDATA

Address offset: 0x480

Data sent from the CPU to the debugger.

Writing to this register will automatically set a DataPending value in the TXSTATUS register.

Bit number				31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
ID				A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A
Reset 0x00000000				0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
ID	R/W	Field	Value ID	Value								Description
A	RW	TXDATA										Data sent to debugger

MAILBOX.TXSTATUS

Address offset: 0x484

This register shows a status that indicates if the data sent from the CPU to the debugger has been read.

Bit number				31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
ID																																			A
Reset 0x00000000				0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
ID	R/W	Field	Value ID	Value								Description
A	R	TXSTATUS										Status of data in register TXDATA
			NoDataPending	0								No data pending in register TXDATA
			DataPending	1								Data pending in register TXDATA

ERASEPROTECT.LOCK

Address offset: 0x500

This register locks the ERASEPROTECT.DISABLE register from being written until next reset.

Bit number				31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
ID																																			A
Reset 0x00000000				0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
ID	R/W	Field	Value ID	Value								Description
A	RW1	LOCK										Lock ERASEPROTECT.DISABLE register from being written until next reset
			Unlocked	0								Register ERASEPROTECT.DISABLE is writeable
			Locked	1								Register ERASEPROTECT.DISABLE is read-only

ERASEPROTECT.DISABLE

Address offset: 0x504

This register disables the ERASEPROTECT register and performs an ERASEALL operation.

Bit number				31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
ID				A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A
Reset 0x00000000				0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
ID	R/W	Field	Value ID	Value								Description
A	RW	KEY										The ERASEALL sequence is initiated if the value of the KEY fields are non-zero and the KEY fields match on both the CPU and debugger sides.