stm32l programming

// two ways to enable clock for port A:

// include name depends on controller
// set up a bit in control register to enable clock port A
#include “stm32l1xx.h”
// this way uses only CMSIS library

// second way is a function call which wraps around a first way
#include “stm32l1xx_rcc.h”
// this way uses standard peripherial library
// note that typical style is not to include “stm32l1xx_rcc.h” directly,
// but instead create project-specific “stm32l1xx_conf.h” file, put include in that file,
// and also define USE_STDPERIPH_DRIVER macro. This way will set stm32l1xx.h to include
// mentioned _conf.h, which, in turn, includes _rcc.h

// two ways of configuring legs 8 and 9 or port B for output in push-pull mode:

#include <stm32l1xx_gpio.h>

// direct write to registers
// requires only CMSIS library (only header “stm32l1xx.h” )

// Each general-purpose I/O port has four 32-bit configuration registers (GPIOx_MODER,
// GPIOx_OTYPER, GPIOx_OSPEEDR and GPIOx_PUPDR), two 32-bit data registers
// (GPIOx_IDR and GPIOx_ODR), a 32-bit set/reset register (GPIOx_BSRR), a 32-bit locking
// register (GPIOx_LCKR) and two 32-bit alternate function selection register (GPIOx_AFRH
// and GPIOx_AFRL).

// GPIOB is a constant of type GPIO_TypeDef, mapped to special address.
// Similar constants are for A and C

// “output” mode means value “01” in corresponding MODER
// 10 – alternate function, 00 – input


// for output and alternate function it is possible to specify open drain or push-pull.
// input always imply push-pull
// if bit is set for pin, then it is open drain
// if bit is reset for pin, then it is push-pull

// for all modes we can specify if it is floating, push up or pull down.
// values are 00 – floating, 01 – push up, 10 – pull down
// following example is for pull down
// following example is for push up
// A better way: modify bits via function.
// requires standard periph driver

// declare and init structure
GPIO_InitTypeDef PORT;
// Specifies mode and speed for a set of pins.
PORT.GPIO_Pin = (GPIO_Pin_9 | GPIO_Pin_8);
PORT.GPIO_Mode = GPIO_Mode_Out_PP;
PORT.GPIO_Speed = GPIO_Speed_2MHz;
// set value for specified port

many ways of setting level 1 on legs 8 and 9 of port C:

// direct write to ‘value’ register of port C.
// get a value, use or-mask to set pins, then write a new value

// write a “set” command to set-reset register of port C.

// via functions
// set several bits to 1
// either this way
GPIO_SetBits(&GPIOC, GPIO_Pin_9 | GPIO_Pin_8);
// or this way
GPIO_WriteBit(&GPIOx, GPIO_Pin_9 | GPIO_Pin_8, Bit_SET);

two ways of setting level 0 on legs 8 and 9 of port C:

// direct write to ‘value’ register of port C.
// get a value, use and-mask to clear pins, then write a new value

// write a “reset” command to set-reset register of port C.

// via functions
// either this way
GPIO_ResetBits(GPIOC, GPIO_Pin_9 | GPIO_Pin_8);
// or this way
GPIO_WriteBit(&GPIOx, GPIO_Pin_9 | GPIO_Pin_8, Bit_RESET);

reading input data via functions

uint8_t GPIO_ReadInputDataBit(GPIOA, uint16_t GPIO_Pin);

Choosing correct alternate function for pin

// first, see above hot to configure a pin by setting MODER to AF (value 10)

// Each pin has several alternate functions. It depends on pin, and on controller
// theoretically up to 16 functions are available. In practice only 2-4 values are used.
// for each port (A, B, C) there is a 64-bit AF register, represented as two 32-bit parts.
// GPIOx_AFRL and GPIOx_AFRH. Each pin has 4 bits in this register.
// So bits for pins 0 – 7 are in GPIOx_AFRL, and bits for pins 8 – 15 are in GPIOx_AFRH
// To understand which pin has which possible values for bits, see datasheet.
// functional way
// for example, pin 6 of port B has timer TIM4 channel1 attached to alternate function 2.
// TIM4 has 4 channels, each attached to different pin. However, all those channels
// are mapped using same alternate function number for each pin, so this alternate
// function is just called ‘GPIO_AF_TIM4’


Configuring timer TIM2:

// first, don’t forget to attach timer to system clock on application bus 1

// each timer has a counter. This counter increments with every timer tick
// it is possible to control a speed of the tick, and a value of counter
// there is also a built-in capability of auto-reset for counter

// prescaler: a divider for system frequency.
// Specifies how fast timer ticks (and counter increments)
TIM2->PSC = 0x00A7;

// auto-reload register. Specifies a value after which timer counter resets
TIM2->ARR = 0x0FFF;
// a frequency of timer reset = sys_freq/[prescaler*(ARR + 1)].

// to start a timer an ‘enabled’ bit should be set in control register CR1

// get a value of counter

Configuring timer TIM2 for capture and compare:

// each timer has several channels, each channel is connected to some output pin
// channels are configured independently

// specify capture and compare value for channel 3
TIM2->CCR3 = 0x050;

// capture and compare mode registers control channels 1,2 (reg CCMR1) 3,4 (reg CCMR2)
// for each channel there are 3 ‘mode’ bits, and they could be set-reset independently
// however, they together specify capture and compare mode.
// following example configures channel 3 and sets bits to 110, which means PWM
// PWM means ‘if TIM2->CNT < TIM2->CCR then out_1 else out_0’

// capture and compare enable register (one for each timer) specifies if use CC for channels
// this example enables capture and compare for channel 3 of timer TIM2
My project: use PWM to control brightness of LED

# Part 1, choosing modules and calculations.

0. Choose brighness. Let b = 50%
1. Find a pin to which LED is connected
2. Find a port for a pin
3. Find if a pin has some timer channel as alternate function.
4. Choose a frequency of PWM. Let f = 1 kHz
5. Let timer’s ARR = 100 – 1, so each tick of timer is 1% of whole period
6. Now we can calculate prescaler.
f = sys_freq/[prescaler*(ARR + 1)]
prescaler = sys_freq/[f*(ARR + 1)] = 24M/[1k*100] = 240

# Part 2, programming

7. Enable system clock for choosen IO port and choosen timer.
8. configure a timer with prescaler and auto-reload register (TIM?->ARR)
9. specify capture-and-compare register for choosen channel with value of b (brightness)
10. for choosen channel find corresponding CCMR register (1 or 2)
and set bits for PWM (bits 2 and 1 are enabled, bit 0 is disabled)
11. configure a pin for output (any frequency) via alternate function, push-pull
12. configure timer as an alternate function for pin
13. Enable CCM for chosen channel via bit in timer’s CCER
14. Enable timer via bit in timer’s CR

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