Search code examples
vb.netmarshallingdllimporti2c

CH341DLL.DLL + I2C not works properly with VB.NET

I write VB.NET class for implement CH341DLL.DLL functionality. The method CH341StreamI2C() is used for stream write and read into device. This way I've imported the method CH341StreamI2C() from DLL: 

<DllImport("CH341DLL.DLL", SetLastError:=True, CallingConvention:=CallingConvention.StdCall)>
Private Shared Function CH341StreamI2C(ByVal iIndex As Integer, ByVal iWriteLength As Integer, ByRef iWriteBuffer As IntPtr, ByVal iReadLength As Integer, ByRef oReadBuffer As IntPtr) As Boolean
End Function

For check how this method works, I use I2C humidity and temperature sensor HTU21D. It's IIC address is 40h, and register where temperature is getting is E3h. So I invoke method CH341StreamI2C() like this:

Dim writeBuffer as Byte() = {&H40, &hE3} 'Address+Command
Dim s As String = Encoding.Unicode.GetString(writeBuffer)
Dim writeBufPtr As IntPtr = Marshal.StringToHGlobalAuto(s) 'Get pointer for write buffer
Dim wLen As Integer = writeBuffer.Length
Dim readBufPtr As IntPtr = IntPtr.Zero 'Init read pointer
Dim rLen as Integer = 3 'Sensor must return 3 bytes
Dim res As Boolean = CH341StreamI2C(0, wLen, writeBufPtr, rLen, readBufPtr)

I use logic analyzer to see what is on the SDA and SCL lines. And result is unpredictable. For example, if call previous code 4 times, that's the result:

Lines SDA and SCL while CH341StreamI2C() calling

It's seen, that physically CH341 device writes unpredictable values in the line. This is not DLL error, because other applications use this method and the result is correct. For note, other methods, e.g. CH341ReadI2C() and CH341WriteI2C(), that reads/writes only one byte per time, acts correct in my code.

What is the probably reason of the such behavior? May be, I've marshalled write buffer incorrect? How is the right way to do this?

Solution

  • If this is what you're using, the original declaration is:

    BOOL WINAPI CH341StreamI2C(ULONG iIndex, ULONG iWriteLength, PVOID iWriteBuffer, ULONG iReadLength, PVOID oReadBuffer);

    Since the buffer parameters are PVOIDs, you should be able to just marshal them directly to byte arrays:

    <DllImport("CH341DLL.DLL", SetLastError:=True, CallingConvention:=CallingConvention.StdCall)>
Private Shared Function CH341StreamI2C(ByVal iIndex As Integer, ByVal iWriteLength As Integer, ByVal iWriteBuffer As Byte(), ByVal iReadLength As Integer, ByVal oReadBuffer As Byte()) As <MarshalAs(UnmanagedType.Bool)> Boolean
End Function

    Arrays are reference types (classes) which means you always refer to them via their memory pointer. Thus when you pass them to a function (P/Invoked or not) you're actually passing the array's pointer, rather than the array itself. This is really helpful when P/Invoking since it often lets you pass the arrays as-is.