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Mastering SPI in STM32 | Register-Level Programming Tutorial #8
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Apr 25, 2021
In this video, we will learn how to use **SPI in STM32 using registers** only ā without HAL or LL libraries. You will see how to configure **SPI registers in STM32** step by step, set up the **GPIO pins for SPI**, and implement **full-duplex communication** to both transmit and receive data. We will also connect the **ADXL345 accelerometer** with STM32 using SPI and read data from it, which makes this a complete **STM32 SPI communication tutorial** at the register level. š **Download the Project Files:** š [https://controllerstech.com/spi-using-registers-in-stm32/] šŗ **Related Videos You Should Watch:** * I2C using Registers in STM32 * UART using Registers in STM32 * ADC using Registers in STM32 #STM32 #SPI #EmbeddedSystems #Microcontroller #Controllerstech #STM32Tutorial #SPItutorial #STM32SPI
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0:09
hello everyone welcome to controllers tech this is another video in the stm 32
0:17
register based series and today we will see how to use the spi in master mode
0:23
i will cover both the transmitting and the receiving part in this video i will write the main code for the f4
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controller but i will explain the changes for the blue pill also later you can download the code for both
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of them before we start let's take a look at the reference manual to see some important information
0:48
as you might know sbi works with four pins meso master enslave out is used by
0:55
master to receive data from the device mossy master out slave in is used by
1:02
master to send the data to the device clock is used to keep the master and slave in sync
1:08
to make sure that the data transmission is correct and slave select pin is used to select
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and unselect the slave device spi can be used in different modes
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we will be using the full duplex mode as it's simpler compared to others
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in full duplex mode all four pins are used and there are separate pins for transmit
1:33
and receive
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whereas in half duplex mode the same line is used for transmit and receive and this makes it a bit more complex
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than the full duplex mode then we have others like transmit only or receive only modes
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since we are doing two-way transmission here these modes are of no use
1:59
the spi can work in multi-slave communication also here we can connect multiple slave
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devices to the same master and later select any device using the
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slave select pin or the nss pin
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we will see rest of these in a while let's start the kyle id and set up our
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program first [Music] i am using f446re here
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select the cm syscor and device header files
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let's add the main file to this project
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and add the main function and a while loop
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first of all we need to copy the clock setup and the delay setup libraries
3:05
i have already covered these you can check out the first two videos in the playlist
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let's copy them inside the project
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folder we also need to include them in our
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project
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and finally include them in the main
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file this is it for the basic setup now let's
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write a function to configure the spi
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here are the steps to do so we enable the clock and configure the
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control registers one and two this is fairly easy
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compared to the i2c or uart let's start by enabling the spi clock
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to do that we need to check the rcc registers
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here the 12 bit of rcc apb b2 enable register controls the spi-1 clock
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let's write a 1 in the 12th position to enable the spi-1 clock
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next we need to configure the control register one
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here we have the spi control register one let's start by configuring the zeroth
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bit of this register
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the zeroth bit configures the clock phase and the first bit configures the clock polarity
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what do these things mean to understand this let's check some description in the
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manual this here is the timing diagram for the
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spi basically the clock polarity means how you want to set the idle clock
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if it is set to polarity one the clock will be ideally high and if set to zero the clock will be
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ideal low and similarly the clock phase is responsible for the where the sampling
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starts notice here that when phase is 1 and polarity is 1
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the data sampling starts at first rising edge and afterwards the data will be always
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sampled at every rising edge of the clock when phase is 1 and polarity is 0
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then the data is sampled at the first falling edge and this will continue similarly these
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are the conditions for the phase zero the settings for polarity and phase depends on the slave device
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in this video i am going to use adxl 345 as a slave device for the
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demonstration i have covered this device in my whole tutorials and this article is one of them here is
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the picture from the data sheet of the adxl as you can see its requirement is that
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the clock polarity and clock phase must be one
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so we will set the bits zero and one to one
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next we have the master mode selection obviously we want the master mode
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so set the bit 2 as 1.
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then we have the board rate control bits here we can choose the board rates as
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per the settings given
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according to the adxl data sheet we can choose the maximum board rate of 5
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megahertz i am using spi-1 which is connected to
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the apb-2 bus
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with the default setup this apb bus is running at 90 megahertz
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if i change this system clock to 160 megahertz the ap b2 peripheral clock will come
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down to 80 megahertz and using this prescaler of 16 the baud
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rate can be 5 megahertz to make these changes we need to go to
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the rcc config file
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and to make that change we will change the value of this plln
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if you pay attention to the clock setup the crystal frequency gets divided by 4 and later by 2 this makes it 1
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megahertz so whatever plln value we use the same will be the system clock
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frequency so i will set it as 160.
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now our apb2 peripheral clock is running at 80 megahertz and to keep the spi 1 clock at 5
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megahertz we will use this prescaler of 16. so we will write 3 in the third position
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next we have the spi enable bit which we will set later in the program
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now configure the frame format here we need to send the msb
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first this is again as per the device requirement
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so we will set the bit 7 as zero
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now comes the ssm and ssi bits software slave management we need to see
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the explanation before configuring these bits here is the nss pin management
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basically what happens is if we set the software slave management to 1 the master can control the slave using
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any i o pin but if the software slave management is off then we must use the nss pin to select
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and unselect the slave device here i will use this software slave
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management and later set any pin as the slave select pin so i am going to keep the ssm bit high
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and ssi bit high 2. let's write 1 to the 8th and 9th positions
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next we have the rx only bit this bit sets the full duplex mode then we have
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data frame bit and here we will use the 8-bit data format next we have set up for the crc which we
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will not be touching today
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and at last these bits are used to configure the receive only mode or transmit only mode
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since we are using full duplex and 8-bit data format we will keep the bit 10 and 11 as zeros
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now let's check the control register 2.
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here we have the configuration for dma and the interrupt since we are not using them we will keep
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the control register 2 as 0.
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this completes the spi configuration this spi configuration remains same for
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the blue pill also now let's configure the gpio
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i am going to use pa5 for the clock pa6 for the miso a7 for the mossy and pa 9 for the slave
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select so let's enable the clock for the gpioa
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it's the 0th bit of ahb1 enable register
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now let's go to the gpio registers and start with the first one that is
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mode register here we will choose the alternate function mode to the pin 5
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pin 6 and pin 7. and the output mode for the pin 9 that's
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the slave select pin here we have to write a 2 in the 10th
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12th and 14th positions also write a 1 in the 18th position
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for the pin pa 9 in the output mode
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next one is output type we will leave it to push pull which is the reset value anyway
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then we have to select the speed we will keep them at very high speed
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again modify all the four pins and write a 3 in the 10th 12th 14th and 18th positions
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we don't need to set up anything here
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now comes the alternate function register we need to first check which alternate
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function is needed to set the spi
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here we have alternate function 5 for the spi-1
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since we are using pins 5 6 and 7 we will be using the af are low register
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if you use pins 8 to 15 you have to use afrh register
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so we will use alternate function 5 for all three pins and to do that we will write a 5th to
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20th 24th and 28th positions
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this is it for the gpio configuration this configuration is slightly different
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in blue pill so let's see that one too
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this is the reference manual for heblew pill same spi pins are used here
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this table 25 directs the configuration for different spi pins as we are configuring the master
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mode we will see the gpio configuration for the same
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the clock pin needs to be configured as alternate function push-pull
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similar configuration for the mossy pin whereas the miso should be set up as the
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input floating or input pull-up
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let's see the registers now for the pin numbers up to 7 we will use
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the configuration register low this register uses four bits to configure each pin
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the first two bits are used for mode and the next two are used for configuring that mode
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pin pa5 is the clock pin and it should be set as alternate function push-pull
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so first we will set it mode as output with maximum speed and then in the
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configuration bits we will configure it as alternate function output push-pull
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here you can see the blue pill code first enable the gpio clock
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before modifying the configuration register we must set it to zero
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that's because its reset state is the floating inputs so this zero one will also get added to
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our values now we will shift the 11 by 20 positions
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[Music] this 11 is because we are writing 1 0 1
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1 in these positions
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similar configuration will be used for mossy pin
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for me so we need to select the input mode
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so basically we just have to write a 1 in the 26th position
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and finally i am using pa4 as the output for the slave select pin
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the s-p-i configuration is same as what we covered in the f-4
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let's go back to our s-p-i functions now
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to enable the spi we will write one in the sixth position in the control
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register one i am defining a new function for this and for disabling the spi we will just
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clear that bit in order to select the slave we have to reset the pin
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pa 9 and to release the slave we will set the pin now let's write a
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function to transmit the data this function will take the pointer to the data
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and the size of the data here are the steps we will wait for the txe bit to set in
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the status register and then transmit the data
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let's check this status register
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bit represents transmit buffer empty if this bit is set it indicates that
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there is no data in the transmit buffer and only then we can load the data into this buffer
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so first we will wait for this bit to set and once it is set we will load the data
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into the data register this data will then be transmitted to the shift register
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and the txe bit will again set we need to perform this before sending each byte
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something important is given here after we write the last data byte there is some small delay before the
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busy flag is set and therefore we must wait for the txe bit to set before checking this busy flag
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so we will wait for the txe bit to set and wait for the busy flag to reset
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this busy flag is 7th bit in the status register and it will be set if the spi is busy
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with communication
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this now is a very important step remember whenever we send the data to
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the spi it sends something in return this is not acknowledgement
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but some random data so before we go on receiving data from the
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slave device we must make sure that the data register is empty
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to do that we will perform a dummy read from the data register also if the overrun flag is set during
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this reception we need to clear it by reading the status register
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this completes the transmit function let's test this part first
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in the main function initialize the clock configure the gpio
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configure the spi enable the spi
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let's define a variable with 3 bytes of data
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spi transmit will transmit these three bytes of data let's build it now
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go to the options and configure the controller
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let's debug it
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i have here the logic analyzer set up in the spi mode
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i think i forgot to enable the chip select
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ok select the slave transmit the data and release the slave
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you can see here this spi is configured as per my setup
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let's enable the recording here run the program now
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this should be enough it is showing
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mismatch but just ignore that you can see the chip select pin goes low before the
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transmission and then goes back high afterwards
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as i told you in the beginning when the clock phase is 1 and clock polarity is 1 the data will
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start sampling at the first rising edge this is the first edge where the
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sampling will start i am going to put these markers in every rising edge
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here we have zero zero zero zero zero zero one zero this is the first byte
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transmitted by the master this corresponds to zero cross zero two
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which is exactly what we sent
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then these bytes will be transmitted the second byte is zero zero one one
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zero zero one zero which is zero cross three two and then the last byte we have
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is zero cross f six the chip select is low the entire time
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also the clock goes to high after the data transmission is finished
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this is because we have selected the clock to be ideal high c pole equals to 1.
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that's how the data is transmitted to the slave device and this part of the program works well
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now let's write another function to receive the data from the slave device
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this function will also take two parameters the pointer to the data and the size of
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the data that you want to receive here are the steps to do so first we
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will wait if the spi is busy in communication then we will send
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some dummy data this dummy data is an important step any spi device only
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transmits data after receiving something so we must send some random data to the
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device now after we send the dummy the ixne flag will be set
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this basically indicates that there is some data in the buffer
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now we will read the data from the data register
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this is all for the spi related functions now we need some device from where we
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can read some data i am going to use the adxl accelerometer
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for this purpose
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these are some functions for the adxl from my earlier tutorials so i am going to modify them
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all we need to do here is change the whole function with our own
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similarly modify the function for adxl [Music]
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read
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we are going to receive 6 bytes from this device
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let's initialize the adxl
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inside the while loop we will read the data from this device i am going to add some delay also
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this delay won't be accurate since i have changed the main clock but this is not the priority right now
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let's build it and see the result on the
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scope
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this much time should be enough
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here we have our result as you can see the data is being received every few
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milliseconds let's check this data now [Music]
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as you can see here we are sending zero cross three one and zero cross zero one
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this data is not clear like i said in the beginning scope is not able to read this first
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byte properly anyway let's check the second byte the transmitted bytes are 0 cross 6 d
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and 0 cross 0 0. this is because we are sending 0 cross 2 d which gets added with 0
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cross bar zero
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we are sending the same thing again but with zero cross zero eight
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our transmit part is okay let's see the receiving part
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we send the address along with some bytes from the device settings to indicate the read operation
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now you can see as we are sending the dummy byte the device is responding with some data
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this process will keep repeating every few milliseconds
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so our device is working perfectly
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i will include some more calculations from the previous code to check the data in the id itself
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here you can see the values from the accelerometers are being shown
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this is actually acceleration in the x y and z axis this video is not about
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accelerometer so i am not explaining this just check the old video
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where i have explained this in detail so this is it for the spi
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we were able to transmit and receive the data from the device you can use many spi devices with a
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single master controlling one device at a time
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to download the code for f4 and blue pill go to the link in the description leave
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comments in case of any doubt keep watching be safe and have a nice
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day ahead
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do
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