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#10. How to configure DMA using Registers || HT and TC Interrupts
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Jun 26, 2021
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View Video Transcript
0:03
[Music]
0:09
welcome to another video of controllers
0:11
tech
0:12
this is yet another video on
0:14
register-based programming series
0:16
and today we will see the dma
0:20
stm32f103c8
0:22
some users reported that they can't get
0:24
the dma to work with the blue pill
0:26
so i decided to make a separate video on
0:28
it i will demonstrate the working of dma
0:32
with the uart
0:33
you art have already been covered with
0:35
f4 series mcus
0:37
and here is the main file from that the
0:40
process is almost
0:41
identical and that's why i will not go
0:44
into very deep today
0:46
you can check out the other video for
0:48
more detailed steps
0:50
we will use the same steps for blue pill
0:53
with the addition of dma of course
0:56
so let's create a project with kyle
0:59
select the cm syscor and the device
1:02
startup files
1:11
here first of all we will include the
1:13
device header file
1:17
now we will write a function to
1:19
configure the uart
1:24
here are the steps to configure the uart
1:27
with dma
1:31
they are identical to that for the f4
1:33
series we discussed earlier
1:35
[Music]
1:37
enable the clocks configure the pins
1:40
enable the uart
1:41
define the word length here we will also
1:45
enable the dma for transmit
1:47
or receive this is the additional step
1:51
then we will set the baud rate and
1:53
enable the transmitter or receiver
2:03
i am going to use the uart 2 so the pins
2:06
pa2
2:07
and pa3 will be used
2:17
here is the entire program for it
2:20
first we will enable the uart clock and
2:23
the gpio clock
2:26
then we will configure the pins for the
2:28
alternate function
2:30
this configuration of pins is a little
2:32
different in the blue pill
2:33
as compared to the f4 series
2:38
let's see it in the reference manual
2:44
we need to check the gpio configuration
2:48
for the device peripherals
2:55
here we have the uart section for
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transmit in the full duplex mode
3:00
the pin must be set as the alternate
3:03
function push-pull
3:05
and for receive it must be set as the
3:07
input floating
3:08
or pull-up
3:11
to set these modes for the pins we will
3:14
check the configuration register
3:17
here we have alternate function output
3:19
push pull 1
3:20
0 and the output mode at maximum speed
3:24
is 1
3:30
1.
3:32
so we are setting the pin p-a-2 as the
3:35
output push-pull
3:37
and the pin p-a-3 in the input mode
3:44
let's see how to enable the internal
3:46
pull-up for the pin
3:52
here is the table as mentioned here
3:56
for the internal pull-up the output data
3:58
register must be high
4:09
next we will enable the uart and then
4:11
configure the dma for the transmit
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or receive or both
4:29
here is the uart control register 3 and
4:32
the bit 7
4:33
and 6 controls the dma
4:43
i am only enabling it for the receive
4:46
next is the setting of the board rate
4:49
the formula
4:50
is the peripheral clock divided by the
4:52
16
4:53
times the usart division if you
4:56
rearrange the formula for the desired
4:58
baud rate
4:59
you will get the values of the mantissa
5:01
and the fraction
5:03
this is explained in the previous video
5:05
so i am not explaining it again here
5:08
anyway this will set up the board rate
5:10
of 115
5:13
200 while the peripheral clock is
5:15
running at 36 megahertz
5:20
you can check which clock is connected
5:22
to the uart in the clock diagram in the
5:25
data sheet
5:32
finally we will enable the transmitter
5:34
and the receiver
5:37
this is it for the uart configuration
5:40
now we will initialize the dma
5:46
before initializing it we need to check
5:48
which channels will we be working with
5:56
here we have the table for dma1 requests
6:00
you can see the uart 2 receive is
6:03
connected to the channel 6 of the dma-1
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so we will write our initialization
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according to that
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first we will enable the dma-1 clock
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now for the rest of the configuration we
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will check the dma registers
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the first register we have is the
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interrupt status register
6:29
the bits here are red only bits and they
6:32
indicate the status of the interrupt
6:35
which was triggered next register is the
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interrupt flag
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clear register and this register is used
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to clear the interrupt bits
6:43
that were set in the previous register
6:46
next we have the channel configuration
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register
6:50
let's start from the least significant
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bit
6:56
the first bit is to enable the dma and
6:59
that we will do in the end
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then the next three bits are interrupt
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enable bits
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and we will enable all three interrupts
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this time
7:09
next is the data direction as we are
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copying data from the uart peripheral to
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the buffer in the memory
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we will keep this bit zero
7:22
next is the bit to enable the circular
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mode and yes we will enable the circular
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mode for the dma
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next bit indicates whether we want to
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increment the peripheral address
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since we are copying the data from the
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data register of the uart
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we don't want to increment the address
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of the peripheral
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but we will increment the memory address
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so that the new data is copied in the
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new location
7:57
next we have the data size for the
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peripheral and the memory
8:01
here we will use 8 bits as that's how
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the uart is set up
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next is the priority for the channel and
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we will keep it low
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this is an important bit if you are
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using more than one channel
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you can control the priority of the
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respective channel
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this is it for the initialization of the
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dma but we still need to configure it
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and we will write a new function for
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that this function takes three
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parameters
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the address of the source destination
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address
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and the size of the data
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the next register we have in line is the
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number of data register
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this register stores the number of data
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item that we want to
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transfer this is a 16 bit register
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so we can transfer up to 65 535 values
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using the dma
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once the transfer is complete the
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register will stay at zero if the normal
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mode is selected
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but if the circular mode is selected the
9:15
value in the register will be auto
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reloaded to the previous value
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and this way transfer never finish
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this is how the circular mode of dma
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actually works
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next is the peripheral address register
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and here we will write the address of
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the peripheral
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in this case the source address since
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our source is the peripheral itself
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next we have is the memory address
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register and in our case
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it will be the destination address this
10:01
will set up everything in the dma
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so finally we will enable the dma
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this is done by writing the bit 0 of the
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control register 1.
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this is it for the dma part now we will
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write a callback function for the dma
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interrupts
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the callback function are defined in the
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startup file
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and we need this one dma1 channel 6
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handler
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once the interrupt is triggered we need
10:47
to check whether it's half transfer
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interrupt or full transfer
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this can be done by checking the dma isr
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register
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here you can see we have bit 22 for the
10:59
half
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transfer interrupt 23 for transfer error
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and 21 for transfer complete
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based on which interrupt is set we can
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do the rest of the processing
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let's define some buffers first
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here if the half transfer interrupt is
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set we will copy the data from rx buffer
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starting from the first position in the
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buffer
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the size of the data will be half of the
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rx buffer
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since that's the data which arrived in
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the buffer
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after copying this data we will clear
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the interrupt bit by writing the
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respective ifcr register value
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this register is used to clear the
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interrupt bit
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set in the isr
12:04
we will update the index and if it is
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more than the main buffer size
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we will reset it
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similarly if the transfer complete
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interrupt bit is set
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we will copy the other half of the rx
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buffer
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after copying the data we will clear the
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transfer complete interrupt flag
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this is a good advantage circular mode
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have
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while we are handling the first half of
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the buffer the second half is receiving
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the data
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even if the data arrives during the
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copying process
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it will be saved in the buffer this way
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the data is never lost
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once all the data is received the dma
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will reset the counter to the original
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value
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and prepare itself for the new data and
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all this happens place without affecting
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the cpu
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now we will write the main function
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let's initialize the system
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as i mentioned this in the previous blue
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pill video
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that by default the mcu will be running
13:17
at 72 megahertz
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and we are okay with it
13:25
now configure the uart initialize the
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dma
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and now we will call the dma
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configuration
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here source address is the address of
13:36
the data register of the uart
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as that's where the data is going to
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arrive
13:41
destination address is the address of
13:43
the rx buffer
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and the size is the size of the rx
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buffer
13:54
i forgot to set the interrupt we will
13:57
set it before configuring the dma
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as the dma will start as soon as the
14:02
configuration ends
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first we will set the priority of the
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dma1
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channel 6 interrupt and then we will
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enable the interrupt
14:28
so that's all now it's the time to test
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it
15:02
i will set a break point in the half
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transfer callback
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i am sending this one 2 3 4.
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for some reason it's not displaying here
15:15
but it's being received
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don't worry about it
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remember that the rx buffer size is set
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to 20 bytes
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so nothing happens until i send at least
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10 bytes of data
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and now we have got a t hit at the break
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point
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dma was set to receive 20 bytes and as
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soon as it received half the data
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the half transfer interrupt was
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triggered
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here you can see all the 10 bytes of
15:44
data
15:45
now let's transfer again
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notice that nothing will happen until we
15:53
receive the full 20 bytes
16:01
another hit at breakpoint and this time
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the transfer complete interrupt was set
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now we will run again and the new data
16:09
has also been copied to the main buffer
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let's try to send the 20 bytes at the
16:18
same time
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something weird is going on with the
16:28
kyle today
16:30
let me try one more time
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it received it this time
16:46
and here you can see the data is copied
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to the main buffer
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also don't worry about those glitches
16:53
you just saw
16:54
it was some connection issue
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let's do one last test and this time i
17:02
will send
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12 bytes
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so we have 12 bytes of data in the rx
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buffer
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but only the 10 bytes were copied to the
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main buffer
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i guess you understand why this happened
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the half transfer interrupt was set
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after the 10 bytes
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so only the 10 bytes will be processed
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but this does not mean that we lost the
17:35
two bytes
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those two bytes will be copied next time
17:40
let's send the same data again
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you can see in the 10th and 11th
17:56
positions
17:57
we have those two bytes which was left
18:00
out the last time
18:05
well that's how it works like i said few
18:08
minutes ago
18:09
the data is never lost in the circular
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dma
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the bytes remain in the buffer and can
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be handled in the next interrupt
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even when we are processing the first
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half of the buffer the data can still be
18:22
saved in the second half
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and this is the entire purpose of having
18:26
half transfer interrupt
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so that we can handle half the data
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while the dma keeps on receiving the
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second half
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this is it for the uart dma the same
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logic
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can be used across all the peripherals
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the dma settings will remain same and
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all you need to configure is the source
18:47
address
18:47
and the destination address the
18:50
peripheral address in almost all cases
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is the data register of the respective
18:54
peripheral
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that's all for the video leave comments
18:59
in case of any doubt
19:01
you can download the code from the link
19:03
in the description
19:05
keep watching and have a nice day ahead
19:28
you
#Programming