STM-SCALER a simple PSC, ARR and CCRx calculator

Usage timescale.py

./timscale.py --help

-f VALUE --freq=VALUE : Calculte ARR and PSC for this FRQUENCY [Hz]!
or
-t VALUE --time=VALUE : Calculte ARR and PSC event inteval [s]!

-b VALUE --bits=VALUE : 16 or 32 - timer type
-c VALUE --clk=VALUE : Timer base clock in [Hz]
-e VALUE --error=VALUE : Accepted error in [%]
-d VALUE --duty=VALUE : Calculate the CRRx value for this duty cycle [%]

If no arguments are given the default values are used. The default values can be seen when starting the program without any arguments.

Example

Finding the first 5 solutions for a Timer Event frequency of 1250 Hz. In addition the CRRx for a duty cycle of 12.5% are calculated. Timer base clock is 84Mhz

./timscale.py --freq 1250 --max=5 --duty=12.5
*** BIT MODE: 16
*** BASE_CLK: 84000000 [Hz]
*** TARGET_FREQUENCY: 1250.0 [Hz]
*** ERROR: 0.0 [%]
*** DUTY: 12.5 [%]
**> MAX: 84000000.0  [Hz]
**> MIN: 0.01955777406692505  [Hz]
(arr+1)*(psc+1) = TIM_BASE_CLOCK / TARGET_F
(arr+1)*(psc+1) = 84000000 / 1250.0
(arr+1)*(psc+1) = 67200.0
Calculate...
PSC: 1 ARR: 33599  => FREQ: 1250.0 [Hz] ERROR: 0.0 [%] - CRRx[ 12.5 %]: 4199
PSC: 2 ARR: 22399  => FREQ: 1250.0 [Hz] ERROR: 0.0 [%] - CRRx[ 12.5 %]: 2799
PSC: 3 ARR: 16799  => FREQ: 1250.0 [Hz] ERROR: 0.0 [%] - CRRx[ 12.5 %]: 2099
PSC: 4 ARR: 13439  => FREQ: 1250.0 [Hz] ERROR: 0.0 [%] - CRRx[ 12.5 %]: 1679
PSC: 5 ARR: 11199  => FREQ: 1250.0 [Hz] ERROR: 0.0 [%] - CRRx[ 12.5 %]: 1399

The problem & solution

For a time based interrupt using the internal clock of a timer finding the right PSC and ARR values for a given interrupt interval (or frequency) can be quite difficult.

Since EVENT_FREQUENCY = TIM_CLK / ( (PSC+1) * (ARR+1) ) finding the correct PSC and ARR values for a given EVENT_FREQUENCY is solving the equation:

(PSC+1) * (ARR+1) = TIM_CLK / EVENT_FREQUENCY = FREQ_RATIO

To solve this equation in a "smart" way we can make some assumtions:

If the FREQ_RATIO is not integer (TIM_CLK modulo FREQ_RATIO) != 0 it is not possible to find a solution for PSC and ARR. In this case a small error must be accepted.
- AND for any given integer PSC the solution of: ARR = (TIM_BASE_CLOCK / (TARGET_F * (PSC+1))) -1 must also be integer. So TIM_BASE_CLOCK modulo (PSC+1) == 0 in order to make ARR as an integer possible.
- AND ARR must be smaler that the maximum of 16-bit: 65535

Since PSC and ARR do have different functionality (ARR is i.e. the range/granularity in which the PWM duty cycle CCRx can be set) its important to have all possible set of PSC and ARR to choose from.

The program gives a list of all possibilities for PSC and ARR.

SMT32 background and timers

Time base generator

Depending on the clock (TIM_CLK), prescaler (PSC) and auto reload (ARR), repetition counter (RCR) parameters, the 16-bit or 32-bit timer can generate an update event from a nanosecond to a few minutes or wider.

TIM_CLK in Hz [1/s]
ETR_CLK in Hz - The external clock frequency connected to ETR pin.
TIx_CLK in Hz - The external clock frequency connected to TI1 pin or TI2 pin.
ITRx_CLK in Hz - The internal trigger frequency mapped to timer trigger input (TRGI)
ETR_PSC - External trigger prescaler starting with 1
PSC - 16-bit value [0:65535] = [0:2^16-1]
ARR - 16-bit value [0:65535] or 32-bit value [0:4294967295] = [0:2^32-1]
RCR - 16-bit value [0:65535] = [0:2^16-1]
EVENT_FREQUENCY in Hz [1/s] between TIM_CLK (max value) and TIM_CLK/(2^16-1)^3 (with RCR) or TIM_CLK/(2^16-1)^2

Internal clock

EVENT_FREQUENCY = TIM_CLK / ( (PSC+1) * (ARR+1) * (RCR + 1))

External mode1 (TI1 or TI2 pins)

EVENT_FREQUENCY = ITIx_CLK / ( (PSC+1) * (ARR+1) * (RCR + 1))

External clock mode2 (ETR pin)

EVENT_FREQUENCY = ETR_CLK / ( (ETR_PSC) * (PSC+1) * (ARR+1) * (RCR + 1))

Internal trigger clock (ITRx)

EVENT_FREQUENCY = ITRx_CLK / ( (PSC+1) * (ARR+1) * (RCR + 1))

Example - Internal clock

TIM_CLK = 84MHz = 84000000 Hz
PSC = 1
ARR = 104
RCR = 0

EVENT_FREQUENCY = 84000000 / ( (1 + 1) * (104 + 1) * (0 + 1)) = 400000 Hz

Exeu/STM32-Scaler