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I'm currently working on a project for which I need to control 16 white LEDs with two shift registers 74HC595 in cascade. I work on a SAMD21 XPLAINED PRO board (µC samd21j18a) and ATMEL Studio as IDE.

I want to send data to the shift registers through SPI. I manage to do it using ATMEL Start to define my configuration but I want to do it with ASF to optimize my code and to get a better control / understanding of it.

Based on samd21j18a datasheet and on different tutorials I found, I tried to do it but for the moment it doesn't work.

#include <asf.h>

#define SLAVE_SELECT_PIN    EXT1_PIN_SPI_SS_0 // Shift register's latch pin on PA05
#define BUFFER_LENGTH       16 // 16 bits message
#define MESSAGE_SIZE        2

uint8_t spi_transfet_array[MESSAGE_SIZE]; // Create a buffer to send through SPI
unsigned char en_pin = IOPORT_CREATE_PIN(1, 9); // Enable pin

struct spi_module spi_master_instance;
struct spi_slave_inst slave;

void spi_configuration(void);
void spi_send_message(uint16_t data);

int main (void)
{
    system_init();
    delay_init();
    spi_configuration();
    
    ioport_set_pin_dir(en_pin, IOPORT_DIR_OUTPUT); // Set enable pin as OUTPUT
    
    while (1) {
        spi_send_message(0b0001100010010000);
    }
}

void spi_configuration(void){
    struct spi_config master_config;
    struct spi_slave_inst_config slave_config;
    
    /*
     * With config by defaults, mux_setting is SPI_SIGNAL_MUX_SETTING_D
     *
     * Based on samd21 datasheet 26.6 "Data In Pinout" :
     * DIPO 0x3 : 
     * - DI on SERCOM0 PAD[3] (PA07)
     *
     * Based on samd21 datasheet 26.7 "Data Out Pinout" :
     * DOP0 0x0 :
     * - DO on SERCOM0 PAD[0] (PA04)
     * - SCK on SERCOM0 PAD[1] (PA05)
     * - SS on SERCOM0 PAD[2] (PA06)
     */
    spi_get_config_defaults(&master_config);
    
    master_config.pinmux_pad0 = EXT1_SPI_SERCOM_PINMUX_PAD0; // MOSI PA04 (Shift register's data pin)
master_config.pinmux_pad1 = EXT1_SPI_SERCOM_PINMUX_PAD1; // SCK PA05 (Shift register's clock pin)
master_config.pinmux_pad2 = EXT1_SPI_SERCOM_PINMUX_PAD2; // Slave Selection PA06 (Shift register's latch pin)
master_config.pinmux_pad3 = EXT1_SPI_SERCOM_PINMUX_PAD3; // MISO PA07 (unused here)
    
    spi_slave_inst_get_config_defaults(&slave_config);
    slave_config.ss_pin = SLAVE_SELECT_PIN;
    
    spi_init(&spi_master_instance, EXT1_SPI_MODULE, &master_config);
    spi_attach_slave(&slave, &slave_config);
    
    spi_enable(&spi_master_instance);
}

void spi_send_message(uint16_t data){
    // Create message to send through SPI
    spi_transfet_array[0] = data >> 8;
    spi_transfet_array[1] = data & 0xFF;
        
    ioport_set_pin_level(en_pin, false); // Set enable pin LOW
    spi_select_slave(&spi_master_instance, &slave, true); // Enable slave selection
    spi_write_buffer_wait(&spi_master_instance, spi_transfet_array, BUFFER_LENGTH); // Send message
    spi_select_slave(&spi_master_instance, &slave, false); // Disable slave selection;
    ioport_set_pin_level(en_pin, true); // Set enable pin HIGH
    delay_ms(1); // Debounce time
    ioport_set_pin_level(en_pin, false); // Set enable pin LOW;
}

EDIT : A schematic of my assembly : Schematic

I've tried to do it in different ways but I don't manage to get a good result. That's the first time I work with SPI and I'm really not sure to do it correctly (especially the mux configuration).

I use the SERCOM0 for which the PADS 0, 1, 2 and 3 are PA04, PA05, PA06 and PA07

My mux configuration is SPI_SIGNAL_MUX_SETTING_D (default mux config) which means DOPO: 0x0, DIPO: 0x3

EDIT : Due to a misreading of the technical documentation I had initially made a connection error which I have now corrected (the problem persists however). See below the new connexions.

Default Mux config

Data In Pinout

Data Out Pinout

I/O Multiplexing

My connexions are :

Data pin (MOSI) : `PA04`,
Enable pin : `PB09`,
Latch pin (SS pin) : `PA06`,
Clock pin (SCK pin) : `PA05`.

Do you have any idea of what could be wrong ?

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  • 1
    \$\begingroup\$ What does "I don't manage to get a good result" mean? Explain what doesn't work as expected and we may be able to help. \$\endgroup\$
    – Finbarr
    Mar 29, 2022 at 17:48
  • \$\begingroup\$ The CD74HC595 will work at 3.3v and less and it should work when connected directly with your microcontroller. However, you may run into some problems if you are using a 5v supply for the cd74HC595 and operating the microcontroller at a lower supply voltage. Also, make sure the ground wires of both are connected to each other. \$\endgroup\$ Mar 29, 2022 at 20:41
  • \$\begingroup\$ @Finbarr : "I don't manage to get a good result" actually means that I don't manage to control the LEDs which are connected to the shift register. I can't really tell you more than that because for the moment my message doesn't seems to be sent correctly to the shift register and the LEDs don't turn on. \$\endgroup\$
    – Fanch
    Mar 30, 2022 at 8:39
  • \$\begingroup\$ @GT Electronics : On CD74HC595 datasheet it's written "Wide operating voltage range of 2 V to 6 V" so I don't reallly understand why it should only work with 3.3V ? Moreover as I say in my question it worked perfectly when I used ATMEL Start to define my configuration. So I don't think that the problem comes from an hardware incompatibility. \$\endgroup\$
    – Fanch
    Mar 30, 2022 at 8:42
  • \$\begingroup\$ @Solomon Slow : Yes, you're right, I think I should use a logic analyzer. I'll probably look into buying one. (Actually i'm looking for it but it's quite expensive...) May be I can do it with the embedded debugger which is on the SAMD21XPLAINED PRO board. \$\endgroup\$
    – Fanch
    Mar 30, 2022 at 9:07

1 Answer 1

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I found the problem... My buffer is constituted of two 8 bits elements and when I send my message through SPI I used to do :

spi_write_buffer_wait(&spi_master_instance, spi_transfet_array, BUFFER_SIZE); 
// With BUFFER_SIZE = 16

but what I had to do is :

spi_write_buffer_wait(&spi_master_instance, spi_transfet_array, MESSAGE_SIZE);
// With MESSAGE_SIZE = 2

I'm now able to control the LEDs I wand to turn on or off with the two shift registers in cascade and through the SPI bus.

The only problem I had was that my LEDs were shining very weakly. To solve this problem I had to decrease my baudrate to 50 000 and the problem was fixed (the default config takes a baudrate at 100 000).

The complete code : (I changed few parameters)

#include <asf.h>

#define SLAVE_SELECT_PIN    EXT1_PIN_SPI_SS_0 // Shift register's latch pin on PA05
#define MESSAGE_SIZE        2

uint8_t spi_transfet_array[MESSAGE_SIZE]; // Create a buffer to send through SPI
unsigned char en_pin = IOPORT_CREATE_PIN(1, 9); // Set enable pin as PB09

struct spi_module spi_master_instance;
struct spi_slave_inst slave;

void spi_configuration(void);
void spi_send_message(uint16_t data);

int main (void)
{
    system_init();
    delay_init();
    spi_configuration();
    
    ioport_set_pin_dir(en_pin, IOPORT_DIR_OUTPUT); // Set enable pin as OUTPUT
    
    while (1) {
        spi_send_message(0b1000000000000001);
    }
}

void spi_configuration(void){
    struct spi_config master_config;
    struct spi_slave_inst_config slave_config;
    

    spi_get_config_defaults(&master_config);
    
    /*
     * With mux_setting is SPI_SIGNAL_MUX_SETTING_E
     *
     * Based on samd21 datasheet 26.6 "Data In Pinout" :
     * DIPO 0x0 : 
     * - DI on SERCOM0 PAD[0] (PA04)
     *
     * Based on samd21 datasheet 26.7 "Data Out Pinout" :
     * DOP0 0x1 :
     * - DO on SERCOM0 PAD[2] (PA06)
     * - SCK on SERCOM0 PAD[3] (PA07)
     * - SS on SERCOM0 PAD[1] (PA05)
     */
    master_config.mux_setting = SPI_SIGNAL_MUX_SETTING_E;
    master_config.pinmux_pad0 = EXT1_SPI_SERCOM_PINMUX_PAD0; // MISO PA04 (unused here)
    master_config.pinmux_pad1 = EXT1_SPI_SERCOM_PINMUX_PAD1; // Slave Selection PA05 (Shift register's latch pin)
    master_config.pinmux_pad2 = EXT1_SPI_SERCOM_PINMUX_PAD2; // MOSI PA06 (Shift register's data pin)
    master_config.pinmux_pad3 = EXT1_SPI_SERCOM_PINMUX_PAD3; // SCK PA07 (Shift register's clock pin)

    master_config.data_order = SPI_DATA_ORDER_LSB;
    master_config.mode_specific.master.baudrate = 50000;
    
    spi_slave_inst_get_config_defaults(&slave_config);
    slave_config.ss_pin = SLAVE_SELECT_PIN;
    
    spi_init(&spi_master_instance, EXT1_SPI_MODULE, &master_config);
    spi_attach_slave(&slave, &slave_config);
    
    spi_enable(&spi_master_instance);
}

void spi_send_message(uint16_t data){
    // Create message to send through SPI
    spi_transfet_array[1] = data >> 8;
    spi_transfet_array[0] = data & 0xFF;
        
    ioport_set_pin_level(en_pin, false); // Set enable pin LOW
    spi_select_slave(&spi_master_instance, &slave, true); // Enable slave selection
    spi_write_buffer_wait(&spi_master_instance, spi_transfet_array, MESSAGE_SIZE); // Send message
    spi_select_slave(&spi_master_instance, &slave, false); // Disable slave selection
    ioport_set_pin_level(en_pin, true); // Set enable pin HIGH
    delay_ms(1); // Debounce time
    ioport_set_pin_level(en_pin, false); // Set enable pin LOW
}
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