5. Backup registers (BKP)

Low-density value line devices are STM32F100xx microcontrollers where the flash memory density ranges between 16 and 32 Kbytes.

Medium-density value line devices are STM32F100xx microcontrollers where the flash memory density ranges between 64 and 128 Kbytes.

High-density value line devices are STM32F100xx microcontrollers where the flash memory density ranges between 256 and 512 Kbytes.

This section applies to the whole STM32F100xx family, unless otherwise specified.

5.1 BKP introduction

The backup registers are ten 16-bit registers in low and medium density devices, 42 registers in high-density devices for storing 20 or 84 bytes of user application data.

They are implemented in the backup domain that remains powered on by V _BAT when the V _DD power is switched off. They are not reset when the device wakes up from Standby mode or by a system reset or power reset.

In addition, the BKP control registers are used to manage the Tamper detection feature and RTC calibration.

After reset, access to the Backup registers and RTC is disabled and the Backup domain (BKP) is protected against possible parasitic write access. To enable access to the Backup registers and the RTC, proceed as follows:

• • enable the power and backup interface clocks by setting the PWREN and BKPEN bits in the RCC_APB1ENR register
• • set the DBP bit in the Power control register (PWR_CR) to enable access to the Backup registers and RTC.

5.2 BKP main features

• • 20-byte data registers (in low and medium-density devices) or 40-byte data registers (in high-density devices)
• • Status/control register for managing tamper detection with interrupt capability
• • Calibration register for storing the RTC calibration value
• • Possibility to output the RTC Calibration Clock, RTC Alarm pulse or Second pulse on TAMPER pin PC13 (when this pin is not used for tamper detection)

5.3 BKP functional description

5.3.1 Tamper detection

The TAMPER pin generates a Tamper detection event when the pin changes from 0 to 1 or from 1 to 0 depending on the TPAL bit in the Backup control register (BKP_CR) . A tamper detection event resets all data backup registers.

However to avoid losing Tamper events, the signal used for edge detection is logically ANDed with the Tamper enable in order to detect a Tamper event in case it occurs before the TAMPER pin is enabled.

• • When TPAL=0: If the TAMPER pin is already high before it is enabled (by setting TPE bit), an extra Tamper event is detected as soon as the TAMPER pin is enabled (while there was no rising edge on the TAMPER pin after TPE was set)
• • When TPAL=1: If the TAMPER pin is already low before it is enabled (by setting the TPE bit), an extra Tamper event is detected as soon as the TAMPER pin is enabled (while there was no falling edge on the TAMPER pin after TPE was set)

By setting the TPIE bit in the BKP_CSR register, an interrupt is generated when a Tamper detection event occurs.

After a Tamper event has been detected and cleared, the TAMPER pin should be disabled and then re-enabled with TPE before writing to the backup data registers (BKP_DRx) again. This prevents software from writing to the backup data registers (BKP_DRx), while the TAMPER pin value still indicates a Tamper detection. This is equivalent to a level detection on the TAMPER pin.

Note: Tamper detection is still active when V _DD power is switched off. To avoid unwanted resetting of the data backup registers, the TAMPER pin should be externally tied to the correct level.

5.3.2 RTC calibration

For measurement purposes, the RTC clock with a frequency divided by 64 can be output on the TAMPER pin. This is enabled by setting the CCO bit in the RTC clock calibration register (BKP_RTCCR) .

The clock can be slowed down by up to 121 ppm by configuring CAL[6:0] bits.

For more details about RTC calibration and how to use it to improve timekeeping accuracy, refer to AN2604 "STM32F101xx and STM32F103xx RTC calibration".

5.4 BKP registers

Refer to Section 1.1 on page 32 for a list of abbreviations used in register descriptions.

The peripheral registers can be accessed by half-words (16-bit) or words (32-bit).

5.4.1 Backup data register x (BKP_DRx) (x = 1 ..20)

Address offset: 0x04 to 0x28, 0x40 to 0x64

Reset value: 0x0000 0000

Bits 15:0 D[15:0] Backup data

These bits can be written with user data.

Note: The BKP_DRx registers are not reset by a System reset or Power reset or when the device wakes up from Standby mode. They are reset by a Backup Domain reset or by a TAMPER pin event (if the TAMPER pin function is activated).

5.4.2 RTC clock calibration register (BKP_RTCCR)

Address offset: 0x2C

Reset value: 0x0000 0000

Bits 15:10 Reserved, must be kept at reset value.

Bit 9 ASOS : Alarm or second output selection

When the ASOE bit is set, the ASOS bit can be used to select whether the signal output on the TAMPER pin is the RTC Second pulse signal or the Alarm pulse signal:

0: RTC Alarm pulse output selected

1: RTC Second pulse output selected

Note: This bit is reset only by a Backup domain reset.

Bit 8 ASOE : Alarm or second output enable

Setting this bit outputs either the RTC Alarm pulse signal or the Second pulse signal on the TAMPER pin depending on the ASOS bit.
The output pulse duration is one RTC clock period. The TAMPER pin must not be enabled while the ASOE bit is set.

Note: This bit is reset only by a Backup domain reset.

Bit 7 CCO : Calibration clock output

0: No effect

1: Setting this bit outputs the RTC clock with a frequency divided by 64 on the TAMPER pin.

The TAMPER pin must not be enabled while the CCO bit is set in order to avoid unwanted Tamper detection.

Note: This bit is reset when the \( V_{DD} \) supply is powered off.

Bit 6:0 CAL[6:0] : Calibration value

This value indicates the number of clock pulses that will be ignored every \( 2^{20} \) clock pulses.

This allows the calibration of the RTC, slowing down the clock by steps of \( 1000000/2^{20} \) PPM.

The clock of the RTC can be slowed down from 0 to 121PPM.

5.4.3 Backup control register (BKP_CR)

Address offset: 0x30

Reset value: 0x0000 0000

Bits 15:2 Reserved, must be kept at reset value.

Bit 1 TPAL : TAMPER pin active level

0: A high level on the TAMPER pin resets all data backup registers (if TPE bit is set).

1: A low level on the TAMPER pin resets all data backup registers (if TPE bit is set).

Bit 0 TPE : TAMPER pin enable

0: The TAMPER pin is free for general purpose I/O

1: Tamper alternate I/O function is activated.

Note: Setting the TPAL and TPE bits at the same time is always safe, however resetting both at the same time can generate a spurious Tamper event. For this reason it is recommended to change the TPAL bit only when the TPE bit is reset.

5.4.4 Backup control/status register (BKP_CSR)

Address offset: 0x34

Reset value: 0x0000 0000

Bits 15:10 Reserved, must be kept at reset value.

Bit 9 TIF : Tamper interrupt flag

This bit is set by hardware when a Tamper event is detected and the TPIE bit is set. It is cleared by writing 1 to the CTI bit (also clears the interrupt). It is also cleared if the TPIE bit is reset.

0: No Tamper interrupt

1: A Tamper interrupt occurred

Note: This bit is reset only by a system reset and wakeup from Standby mode.

Bit 8 TEF : Tamper event flag

This bit is set by hardware when a Tamper event is detected. It is cleared by writing 1 to the CTE bit.

0: No Tamper event

1: A Tamper event occurred

Note: A Tamper event resets all the BKP_DRx registers. They are held in reset as long as the TEF bit is set. If a write to the BKP_DRx registers is performed while this bit is set, the value will not be stored.

Bits 7:3 Reserved, must be kept at reset value.

Bit 2 TPIE : TAMPER pin interrupt enable

0: Tamper interrupt disabled

1: Tamper interrupt enabled (the TPE bit must also be set in the BKP_CR register)

Note: A Tamper interrupt does not wake up the core from low-power modes.

This bit is reset only by a system reset and wakeup from Standby mode.

Bit 1 CTI : Clear tamper interrupt

This bit is write only, and is always read as 0.

0: No effect

1: Clear the Tamper interrupt and the TIF Tamper interrupt flag.

Bit 0 CTE : Clear tamper event

This bit is write only, and is always read as 0.

0: No effect

1: Reset the TEF Tamper event flag (and the Tamper detector)

5.4.5 BKP register map

BKP registers are mapped as 16-bit addressable registers as described in the table below:

Table 14. BKP register map and reset values

Table 14. BKP register map and reset values (continued)

Refer to Table 1 on page 37 and Table 2 on page 38 for the register boundary addresses.