File content as of revision 0:25c34018702c:

#include "mbed.h"

// Hardware Quadrature Encoder ABZ for Nucleo F401RE
// Output on debug port to host PC @ 9600 baud
// 
// By Nigel Webb, November 2014

/* Connections
   PA_0 = Encoder A
   PA_1 = Encoder B
   PA_4 = Encoder Z 
*/

InterruptIn ZPulse(PA_4) ; // Setup Interrupt for Z Pulse

void EncoderInitialise(void) {
    // configure GPIO PA0 & PA1 as inputs for Encoder
    RCC->AHB1ENR |= 0x00000001;  // Enable clock for GPIOA
 
    GPIOA->MODER   |= GPIO_MODER_MODER0_1 | GPIO_MODER_MODER1_1 ;           //PA0 & PA1 as Alternate Function   /*!< GPIO port mode register,               Address offset: 0x00      */
    GPIOA->OTYPER  |= GPIO_OTYPER_OT_0 | GPIO_OTYPER_OT_1 ;                 //PA0 & PA1 as Inputs               /*!< GPIO port output type register,        Address offset: 0x04      */
    GPIOA->OSPEEDR |= GPIO_OSPEEDER_OSPEEDR0 | GPIO_OSPEEDER_OSPEEDR1 ;     // Low speed                        /*!< GPIO port output speed register,       Address offset: 0x08      */
    GPIOA->PUPDR   |= GPIO_PUPDR_PUPDR0_1 | GPIO_PUPDR_PUPDR1_1 ;           // Pull Down                        /*!< GPIO port pull-up/pull-down register,  Address offset: 0x0C      */
    GPIOA->AFR[0]  |= 0x00000011 ;                                          //  AF01 for PA0 & PA1              /*!< GPIO alternate function registers,     Address offset: 0x20-0x24 */
    GPIOA->AFR[1]  |= 0x00000000 ;                                          //                                  /*!< GPIO alternate function registers,     Address offset: 0x20-0x24 */
   
    // configure TIM2 as Encoder input
    RCC->APB1ENR |= 0x00000001;  // Enable clock for TIM2
 
    TIM2->CR1   = 0x0001;     // CEN(Counter ENable)='1'     < TIM control register 1
    TIM2->SMCR  = 0x0003;     // SMS='011' (Encoder mode 3)  < TIM slave mode control register
    TIM2->CCMR1 = 0xF1F1;     // CC1S='01' CC2S='01'         < TIM capture/compare mode register 1
    TIM2->CCMR2 = 0x0000;     //                             < TIM capture/compare mode register 2
    TIM2->CCER  = 0x0011;     // CC1P CC2P                   < TIM capture/compare enable register
    TIM2->PSC   = 0x0000;     // Prescaler = (0+1)           < TIM prescaler
    TIM2->ARR   = 0xffffffff; // reload at 0xfffffff         < TIM auto-reload register
  
    TIM2->CNT = 0x0000;  //reset the counter before we use it  
}

// Z Pulse routine
void ZeroEncoderCount() {
    TIM2->CNT=0 ; //reset count to zero
}

int main() {
    EncoderInitialise() ;
    
    ZPulse.rise(&ZeroEncoderCount) ; //Setup Interrupt for rising edge of Z pulse 
    ZPulse.mode(PullDown) ; // Set input as pull down
     
    unsigned int EncoderPosition ;
    
    while (true) {
        // Print Encoder Quadrature count to debug port every 0.5 seconds
        EncoderPosition = TIM2->CNT ; // Get current position from Encoder
        printf("Encoder Position %i\r\n", EncoderPosition); 
        wait(0.5);
    }
   
       
}