Serial port transmission speed and the R1 prerelease

I am working trying to better understand comports and the maximum data transmission rates. The results I am seeing are puzzling.
Using a FEZraptor, and simply tying the input to the output of two different comports using simple jumpers and a breakout module.
Running the latest prelease from GHI.
I measure the following using the debug statements, and a logic analyzer:
At 921600 baud, 24.13 frames per seconds, 43.66mS between end of frame and start of next frame, frame time = 750uS
At 115200 baud, 7.86 frames per second, frames broken in the 15 character blocks, 21 mS gap between blocks, 43mS gap between frame end and start
At 9600 baud, 1.42 frames per second, frames brokent into 2 character blocks, 21 ms gap between blocks, 43mS gap between frame end and start

For reference, a baudrate of 115200 should results in a maximum of (115200 / 10 / 63) = `82 frames / second
While I would not expect that due to overhead, I would not expect to get less than 10% of that. I also would not expect the frames to be fractured.

Any ideas would be appreciated.

Imports GT = Gadgeteer
Imports Gadgeteer.SocketInterfaces

Namespace SerialTest
    Partial Public Class Program
        Public baud As Integer = 9600 ' 115200 ' 921600
        Dim starttime As DateTime
        Private frames As Integer = 0

        Public Sub ProgramStarted()
            Debug.Print("Program Started")
            _rs232.Configure(baud, SerialParity.None, SerialStopBits.One, 8, HardwareFlowControl.NotRequired)
            _rs2322.Configure(baud, SerialParity.None, SerialStopBits.One, 8, HardwareFlowControl.NotRequired)
            Dim test As New Thread(AddressOf Test1)
        End Sub

        Dim teststring As String = "01234567890ABCDEFGHIJKLMNOPQRSTUVWXYZabcedfghijklmnopqrstuvwxyz"

        Private Sub Test1()
            Debug.Print("Start test1 at " & baud & " baud")
            Dim buffer() As Byte = New Byte(teststring.Length - 1) {}
            _rs232.Port.Write(teststring)  ' send from first comport to second via loopback
            starttime = DateTime.Now
                If _rs2322.Port.BytesToRead >= teststring.Length Then
                    _rs2322.Port.Read(buffer, 0, _rs2322.Port.BytesToRead)  ' reading from second comport
                    frames += 1
                    If frames Mod 10 = 0 Then displayrate()
                    _rs232.Port.Write(teststring)  ' write from first comport
                End If
        End Sub

        Private Sub displayrate()
            Dim et As Double = (DateTime.Now - starttime).Ticks / TimeSpan.TicksPerSecond
            Debug.Print(et.ToString("D") & " seconds or " & (frames / et).ToString("D"))
        End Sub
    End Class
End Namespace

@ Gus - this is in VB just for you :stuck_out_tongue:

And, if I add the line: ManufacturerUse.Configure(0, New Byte() {0})
at the start of the program, 9600 baud speeds up to 16.27 frames per second, without any noticeable gaps in the transmissions.

That secret line disables the dma. Story short, the processor has a horrible uart with a single byte buffer. And for dma it is worse with no useful registers. So, we work around its problems by working the dma periodically, using interrupts is not an option! The other option is to use that magic manufacture line that reverts to the interrupt-no-dma option, with less latencies but then you can easily lose data since it is a single byte buffer!