Event using FTD2XX_NET.dll

Question

I am using a FT232RL chip with FTD2XX_NET.dll I've made a program which writes and reads data to/from AVR atmega32 mcu. First writes data, then reads data as answer.

Now, i want to make an event which indicated me if there's available unreaded data, only when AVR sends data to FTDI buffer and ONLY then. Whithout forcing my program to making loops for checking available data. For my purpose, i want to do the mcu to sends data only when he wants, and the PC must to knows when there's new data in FTDI buffer's chip

Answer 1

Well, first off, from the PC side, it is impossible to know when the AVR sends data to the FTDI chip, you only can know when the FTDI sends its data to the PC. I'm going to assume that is what you meant.

I don't remember what all functions are in the .NET wrapper for the D2XX interface, but in the raw ftd2xx.dll there is a function FT_SetEventNotification which will do what you want. I'm sure it's exposed in the .NET wrapper somehow or another.

Anyways you create an EventWaitHandle, pass it to that function, and as long as your device is opened, the FTDI driver will set it as signaled when a character arrives. Your program, or your reading loop, waits on that handle, then when a character arrives, you read whats in the buffer and parse away.

Keep in mind, you are still going to have to loop over and over while reading data, but what the event signaling is mostly good for is taking it easy on CPU usage.

Answer 2

The D2XX API basically is a way to make the Win32 File API available on other OSes. All its functions try to work exactly like the Win32 APIs.

A number of programmers have learned an inferior approach to communications, usually by bad examples written by framework programmers who don't actually use serial ports on a daily basis. Ask yourself which of these conversations is better:

• > Tell me about incoming data.
• < Ok.
• < EVENT! Here's some data that just arrived.
• > Thanks! Please tell me if you receive any more.

vs

• > Tell me about incoming data.
• < Ok.
• < EVENT! I heard something!
• > What did you hear?
• < Here's the received data.
• > Thanks! I'm ready to hear about more data.

Clearly the first version is a lot simpler and more efficient. That's why I discourage you from asking "How do I get that 'I heard something!' event?" Instead you should focus on how to get the "Here's the new data" event.

If you want an event when your serial-connected device sends you data, use FT_W32_ReadFile in overlapped mode. When you do that, the OVERLAPPED structure has an hEvent member, that event will be triggered when data arrives¹. And the data itself will already be in your buffer, you won't need a separate operation to get it.

The .NET class AutoResetEvent is the way to create a kernel event object that you attach to the OVERLAPPED hEvent.

¹ Actually, when the read operation completes. It can complete because the buffer was filled, or because a timeout (configured using FT_W32_SetCommTimeouts) occurred. The ReadIntervalTimeout is fantastic, because it causes the read to finish whenever there is a gap in the data. This generally corresponds to individual messages from the attached device. While you shouldn't rely on one read completion = one message, the fact is that having an event at the end of a message is the optimal combination of buffering and responsiveness.