Some theory about Systick Timer (RP2040):
reference: https://datasheets.raspberrypi.com/rp20 ... asheet.pdf
Rp2040 Processor features :
• An ARMv6-M compliant 24-bit SysTick timer.
A 24-bit SysTick system timer, extends the functionality of both the processor and the NVIC and provides:
• A 24-bit system timer (SysTick).
• Additional configurable priority SysTick interrupt.
The SysTick timer uses a 1μs pulse as a clock enable. This is generated in the watchdog block as timer_tick. Accuracy
of SysTick timing depends upon accuracy of this timer_tick. The SysTick timer can also run from the system clock (see
SYST_CALIB).
In my project I used the processor clock - 200 MHz.
2 ^ 24 bits = 16,777,216 counts
Systick timer frequency = 200 MHz (period = 5 ns)
Maximum timer count = 16,777,216 * 5 ns =~ 83 miliseconds
Therefore the timer is reset at the beginning of each process.
• func autoZeroADC() // read Zero Volts value
• func dualSlopeADC() // read VIN voltage
SyStick timer
2.4.4.1. System control register summary:
SYST_CSR = SysTick Control and Status Register
Use the SysTick Control and Status Register to enable the SysTick features
Selects the SysTick timer clock source:
0 = External reference clock.
1 = Processor clock.
SYST_RVR = SysTick Reload Value Register
Use the SysTick Reload Value Register to specify the start value to load into the current value register when the
counter reaches 0. It can be any value between 0 and 0x00FFFFFF. A start value of 0 is possible, but has no effect
because the SysTick interrupt and COUNTFLAG are activated when counting from 1 to 0. The reset value of this
register is UNKNOWN.
To generate a multi-shot timer with a period of N processor clock cycles, use a RELOAD value of N-1. For example,
if the SysTick interrupt is required every 100 clock pulses, set RELOAD to 99
SYST_CVR = SysTick Current Value Register
Use the SysTick Current Value Register to find the current value in the register. The reset value of this register is
UNKNOWN.
SYST_CALIB = SysTick Calibration value Register
Use the SysTick Calibration Value Register to enable software to scale to any required speed using divide and
multiply.
reference: https://datasheets.raspberrypi.com/rp20 ... asheet.pdf
Rp2040 Processor features :
• An ARMv6-M compliant 24-bit SysTick timer.
A 24-bit SysTick system timer, extends the functionality of both the processor and the NVIC and provides:
• A 24-bit system timer (SysTick).
• Additional configurable priority SysTick interrupt.
The SysTick timer uses a 1μs pulse as a clock enable. This is generated in the watchdog block as timer_tick. Accuracy
of SysTick timing depends upon accuracy of this timer_tick. The SysTick timer can also run from the system clock (see
SYST_CALIB).
In my project I used the processor clock - 200 MHz.
2 ^ 24 bits = 16,777,216 counts
Systick timer frequency = 200 MHz (period = 5 ns)
Maximum timer count = 16,777,216 * 5 ns =~ 83 miliseconds
Therefore the timer is reset at the beginning of each process.
• func autoZeroADC() // read Zero Volts value
• func dualSlopeADC() // read VIN voltage
SyStick timer
2.4.4.1. System control register summary:
SYST_CSR = SysTick Control and Status Register
Use the SysTick Control and Status Register to enable the SysTick features
Selects the SysTick timer clock source:
0 = External reference clock.
1 = Processor clock.
SYST_RVR = SysTick Reload Value Register
Use the SysTick Reload Value Register to specify the start value to load into the current value register when the
counter reaches 0. It can be any value between 0 and 0x00FFFFFF. A start value of 0 is possible, but has no effect
because the SysTick interrupt and COUNTFLAG are activated when counting from 1 to 0. The reset value of this
register is UNKNOWN.
To generate a multi-shot timer with a period of N processor clock cycles, use a RELOAD value of N-1. For example,
if the SysTick interrupt is required every 100 clock pulses, set RELOAD to 99
SYST_CVR = SysTick Current Value Register
Use the SysTick Current Value Register to find the current value in the register. The reset value of this register is
UNKNOWN.
SYST_CALIB = SysTick Calibration value Register
Use the SysTick Calibration Value Register to enable software to scale to any required speed using divide and
multiply.
Code:
const MAX_RELOAD = 0xFFFFFF // Maximum reload value (24 bits): 16,777,216 ticks 0xFFFFFF// Enable SysTick counterfunc enableSysTick() {rp.PPB.SetSYST_RVR_RELOAD(MAX_RELOAD) // Set maximum reload value (24 bits)rp.PPB.SetSYST_CSR_CLKSOURCE(1) // SysTick clock source: processor clockrp.PPB.SetSYST_CSR_ENABLE(1) // Enable SysTick counterrp.PPB.SetSYST_CVR_CURRENT(0) // Clear current value (24 bits)}Statistics: Posted by Gustavo_Murta — Fri Sep 26, 2025 8:50 pm