System Tick (SysTick) Timer

The System Tick (SysTick) Timer is an integral part of the CPU.

Features

• A 24-bit decrementing, wrap-on-zero, clear-on-write counter

• Dedicated exception vector

• Clocked internally by the system clock

• Secure and non-secure SysTick timers are functionally identical. Each has its own registers and its access depending on the current security state.

Functional Description

SysTick timer block diagram is illustrated below.

The SysTick timer is a 24-bit timer that counts down to zero and generates an interrupt.

1. When enabled, the counter counts down from the value in SYST_CVR. When it reaches zero, SYST_CVR is reloaded with the value held in SYST_RVR on the next clock edge.

2. Reading SYST_CVR returns the value of the counter at the time of the read access.

3. When the counter reaches zero, it sets SYST_CSR.COUNTFLAG to 1. Reading SYST_CSR.COUNTFLAG clears it to 0.

4. A write to SYST_CVR clears both SYST_CVR and SYST_CSR.COUNTFLAG to 0. SYST_CVR is then reloaded with the value held in SYST_RVR on the next clock edge.

Since the SysTick timer is a part of the CPU, it facilitates porting of software by providing a standard timer that is available on ARM Cortex-M based devices. The SysTick timer can be used for:

• a RTOS tick timer which fires at a programmable rate (for example 1000Hz) and invokes a SysTick routine.

• a high-speed alarm timer using the core clock.

• a simple counter. Software can use this to measure time completed and used.

• an internal clock source control based on missing/meeting durations. The COUNTFLAG bit in the SYST_CSR register can be used to determine if an action completed within a set duration, as part of a dynamic clock management control loop.

Refer to the appropriate ARM Cortex User Guide for more details.

Registers

The SysTick timer registers are located on the private peripheral bus of each CPU. The physical base address is 0xE000_E000.

Name	Offset	Access	Description
SYST_CSR	0x010	R/W	System Timer Control and Status Register
SYST_RVR	0x014	R/W	System Timer Reload Value Register
SYST_CVR	0x018	R/W	System Timer Current Value Register
SYST_CALIB	0x01C	RO	System Timer Calibration Value register

SYST_CSR

• Name: System Timer Control and Status Register

• Size: 32 bits

• Address offset: 0x010

• Read/write access: read/write

This register contains control information for the SysTick timer and provides a status flag. It is a part of the CPU, and determines the clock source for the SysTick timer.

Bit	Symbol	Access	Reset	Description
[31:17]	RSVD	-	-	Reserved. Read value is undefined, only zero should be written.
[16]	COUNTFLAG	R/W	0	Returns 1 if the SysTick timer counted to 0 since the last read of this register.
[15:3]	RSVD	-	-	Reserved. Read value is undefined, only zero should be written.
[2]	CLKSOURCE	R/W	0	SysTick clock source selection. 1: The system clock is selected. 0: The output clock from the SysTick clock divider (SYSTICKCLKDIV) is selected as the reference clock. Note When the SysTick clock divider is selected as the clock source, the CPU clock must be at least 2.5 times faster than the divider output.
[1]	TICKINT	R/W	0	SysTick interrupt enable. 1: The SysTick interrupt is enabled. When enabled, the interrupt is generated when the SysTick counter counts down to 0. 0: The SysTick interrupt is disabled.
[0]	ENABLE	R/W	0	SysTick counter enable. 1: The counter is enabled. 0: The counter is disabled.

SYST_RVR

• Name: System Timer Reload Value Register

• Size: 32 bits

• Address offset: 0x014

• Read/write access: read/write

This register is set to the value which is loaded into the SysTick timer, whenever it counts down to zero. This register is loaded by software as part of timer initialization. The SYST_CALIB register may be read and used as the value for SYST_RVR register if the CPU is running at the frequency intended for use with the SYST_CALIB value.

Bit	Symbol	Access	Reset	Description
[31:24]	RSVD	-	-	Reserved. Read value is undefined, only zero should be written.
[23:0]	RELOAD	R/W	0	The value which is loaded into the SysTick counter when it counts down to 0.

SYST_CVR

• Name: System Timer Current Value Register

• Size: 32 bits

• Address offset: 0x018

• Read/write access: read/write

This register returns the current count from the SysTick counter when read by software.

Bit	Symbol	Access	Reset	Description
[31:24]	RSVD	-	-	Reserved. Read value is undefined, only zero should be written.
[23:0]	CURRENT	R/W	0	Reading this filed gets the current value of the SysTick counter. Writing any value clears the SysTick timer counter for the selected Security state to zero.

SYST_CALIB

• Name: System Timer Calibration Value Register

• Size: 32 bits

• Address offset: 0x01C

• Read/write access: read-only

The value of this register is provided by the value of the SYST_CALIB register in the system configuration (SYSCON) block.

Bit	Symbol	Access	Reset	Description
[31]	NOREF	R	0	Indicates whether an external reference clock is available. This bit is loaded from the SYST_CALIB register in SYSCON. 0: A separate reference clock is available. 1: A separate reference clock is not available.
[30]	SKEW	R	0	Indicates whether the TENMS value generates a precise 10-millisecond time, or an 0 approximation. This bit is loaded from the SYST_CALIB register in SYSCON. 0: The value of TENMS is considered to be precise. 1: The value of TENMS is not considered to be precise.
[29:24]	RSVD	-	-	Reserved. Read value is undefined, only zero should be written.
[23:0]	TENMS	R	0	This field is loaded 0 from the SYST_CALIB register in SYSCON, and always RAZ/WI (read as zero, write ignored).

SysTick Timer Configuration

The SysTick timer provides a fixed 1 millisecond time interval for operating system or other system management software. The SysTick timer is clocked from the CPU clock (default is the system clock). In order to generate recurring interrupts at a specific interval, the SYST_RVR register must be initialized with the correct value for the desired interval.

Here is an example of 1ms interrupt interval clocked from the system clock. If the system clock is 200MHz, then the SysTick configuration is:

1. Calculate and Set the SYST_RVR = (system clock(HZ) * interrupt interval(s)) ─ 1 = 200000000/1000 ─ 1 = 0x30d3f.

2. Write SYST_CVR to any value to clear SYST_CVR to zero and clear SYST_CSR.COUNTFLAG.

3. Program the SYST_CSR is 0x7 to select system clock, enable SysTick interrupt and enable count.