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Control, transfer, control, transfer in USB is a bit different from the other transfer types we already

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explained.

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Every one of the other TransFair types we talked about consists of only a single transaction.

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But control, transfer or every control, every every controlled TransFair consists of multiple USV

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transactions, and these transactions are divided into three stages.

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So let's now talk about these three stages.

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But before starting, let me tell you this.

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Gray packets are packets sent by the host and white packets are packets sent by the device.

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So let's start explaining the first stage in every single control transfer setup stage setup stage is

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always the first stage in all controlled transfers and set up stage consists of one single USB transaction

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and just it's a normal transaction, which consists of three packets as we see here, token packet,

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the data packet, then handshake packet.

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The token packet in this setup stage is a setup token and this is the first time we see setup token

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inus.

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And this is probably the only place where you will see a set up tour continues and as and if you remember,

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we already said that set up Tolkan is very similar to out Tolkan.

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So it just indicates that the host will be sending some data to the device.

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And this is exactly what we see here.

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And when the device receives all the data successfully, it acknowledged that by sending acknowledged

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handshake.

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So again, the first stage in every single control transfer is set up stage and set up stage consists

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of only one single USB transaction.

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It's a set up transaction because the token of this transaction is set up token and this setup token

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tells the device that the host will be sending some special data.

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And here this sentence is very important.

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Let me repeat this again.

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Set up token, unlike out token setup, Token tells the device that the host will be sending some special

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data or let's say some special pre structured data in the data packet.

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And this special pre structured data is called USPI request.

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In simple words, set up stage is responsible for sending a USB request from the host to the device.

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And this request, or let's say that the details of this request exist inside the data packet of the

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setup stage.

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And this request may be asking the device to receive some data from the host or it may be asking the

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device to send some data to the host.

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It depends on the request itself.

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So let me repeat this one last time again, set up stage is the very first stage in every single control

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TransFair.

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And it consists of one single transaction, and this transaction is a set up transaction because it

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begins when the host sends a set up token.

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This set token tells the device that in the upcoming data packet, there will be a request coming from

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the host.

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And this USB request is just let's say it's just normal data, but it's pre structured.

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So every single bit of this data means something special.

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We will see the structure and details of the important U.S. request soon.

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And finally, when the device receives that this request successfully, it acknowledged by sending an

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AK hansei packet.

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Hopefully this is clear.

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Now, just before moving to the next stage of the control transfer.

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You probably noticed that I didn't draw or mention nacke and total handshake packets here.

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This is simply because USB devices or let's say more accurately from now, add up USB functions are

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not allowed to reply to set up tokens by Mac or Stoel handshake.

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The device must do its its best to be able to always receive set up tokens.

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And if something wrong happened, the device can only ignore what it received without sending any handshake

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packet.

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This is why I didn't mention Mac and stole packets here.

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The second stage in USV Control TransFair is data stage.

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Data stage is an optional stage, which means it may present in some but not all of controlled transfers.

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And if control transfer includes data stage, this data stage may consist of one to multiple USB transactions.

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So unlike setup stage, which consists only of a single USB transaction, data stage can consist of

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multiple USB transactions.

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And if a control transfer has a data stage or not, and how many transactions data stage should have,

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that totally depends on the USB requests in the setup stage.

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So in simple words, the USB request may be asking the device to send some data to the host, or it

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may be telling the device to receive some data from the host.

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In such cases, data stage will exist and how many transactions it will include.

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This totally depends on how big the data the host when the host wants to send or receive to or from

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the device.

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So if the host in the USB request in the setup stage is asking the device to send some data to the host,

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then the transaction in the data stage will be in data transactions.

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So this here.

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Because the data will be flowing from the device to the host.

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On the other hand, if the host in the U.S. request is requesting the device or is requesting from the

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device to receive some some data from the host, then data stage will include out transactions.

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Like this here.

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So maybe single or multiple out transactions and the same year, maybe one or multiple in transactions.

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This totally depends on the U.S. request itself and on the size of data that will be sent or received,

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and here, as you can see this in transaction or this transaction or.

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Just very normal in and all transactions, just like what we already saw before in the other transfer

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types.

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So in the data stage, when the host wants to receive some data from the device, it sends an Intercon.

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Then the device starts sending the data and the host acknowledged receiving the data when everything

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is received or was received successfully.

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And here the device has again the possibility to reply by Nacke or total handshake packets, and similarly,

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if the host wants the device to receive some data, it sends in the data stage and out packet or outtalking,

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let's say.

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Then it sends the data and the device acknowledges receiving the data.

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If everything was received successfully, so very normal in or out transactions.

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Let me remind you again here.

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I'm just showing you one transaction in both cases.

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So one in transaction in case the host requests receiving some data and one transaction in case the

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host is requesting the device to receive some data.

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But the stage may include not only one transaction, but multiple transactions.

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So we can have multiple in transactions in the data stage or multiple out transactions in that data

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stage.

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This is, of course, if data packet is too small to send the whole data in only a single packet, then

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sending multiple transactions is required.

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And finally, the last stage in TransFair is status stage.

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Status stage includes only one single USB transaction, and this transaction here in the status stage

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is always the opposite of the data stage.

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What does this mean?

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This means if the data in the data stage was sent in the indirection.

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Then the stage, the stage will be an outdoor transaction and similarly, if the direction of the data

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in the data stage was out.

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Then the status stage will be and in transaction, and this makes total sense because here if the host

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in the setup stage requested some data from the device.

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Then the device will be sending some data in the data stage.

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And when the host receives the whole data, it will tell that by sending the opposite transaction,

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so the transactions in that data stage where in transactions then the host will finish the whole control

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transfer by sending an out transaction.

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And similarly, if in the setup stage, if the host requested the device to receive some data, then

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the host will be sending the data in the data stage.

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And when the host finishes sending the whole data, it will tell the device that the whole data has

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been sent by sending the opposite transaction, which is an in transaction.

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And the data packet in the state of stage has no data in it, it's a zero Leontes data packet, which

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means it has no payload.

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If the transaction in the state stage was an out transaction or in transaction, it doesn't matter.

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Data packet in status stage includes no data.

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It has no payload because it's just a signal to tell the host or the device that control transfer has

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just finished.

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Hopefully, you understand now USB control transfers, because this is a very important topic here and

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we will spend a lot of time in implementing this principle and this figure here or the information or

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the mechanism you see here in this figure.

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So please try to understand it as good as possible.