Instruction cycle measurement

[Christopher4187]

I have a program on an 18F4550 with a 20 mhz oscillator that interfaces with a VB program via USB. I need to be able to monitor data at about 500Kbps. I'm using the computer to store the information as it comes in. When I use the computer in real time (not storing data but just viewing) it seems pretty quick. When I press the button to store the data, it seems to slow just a little bit. When I look at the data using Excel, I have roughly 45 measurements per second, which is about 455 short of what I need. I think the 18F4550 can achieve a 2mS measurement but I'm not 100% sure since I've never needed anything with this short of a timing cycle.

So my questions are:

1. Can the 18F4550 do this quick of a measurement?

2. Is there a list of instructions where it lists the amount of time (i.e. If then = 400nS)

3. Is there an easy way to find out where my issue is? (VB or PBP code)

4. Is there a relatively easy way to do data logging on a PIC? Would I need an external memory chip?

[enauman]

Look at the data sheet for the A/D converter. I assume you are making analog measurements. Most PICs A/D can sample around 50uS.

[HenrikOlsson]

Hi,
How do you transfer the data from the PIC to the PC?

I'm a bit confused about your math... You say you need 500Kbs (which is quite a bit), but then you say you want 500 measurements per second. With an ADC resolution of 10 bits (each measurement taking two bytes) that's 1000 bytes per second which would be around 100Kbs. Never the less, the bottleneck is probably in the datatransfer itself.

Is it a continous stream of samples or is it a defined amount of samples at a certain sample rate or what exactly are you trying to do. The 4550 has 2k of ram so depending on what else your program is doing you might be able to store the samples in RAM and then transfer them to the PC - but again, it all depends on what you're doing.

/Henrik.

[Christopher4187]

I assume you are making analog measurements.

Your assumption is wrong.

How do you transfer the data from the PIC to the PC?

I'm a bit confused about your math... You say you need 500Kbs (which is quite a bit), but then you say you want 500 measurements per second. With an ADC resolution of 10 bits (each measurement taking two bytes) that's 1000 bytes per second which would be around 100Kbs. Never the less, the bottleneck is probably in the datatransfer itself.

Is it a continous stream of samples or is it a defined amount of samples at a certain sample rate or what exactly are you trying to do. The 4550 has 2k of ram so depending on what else your program is doing you might be able to store the samples in RAM and then transfer them to the PC - but again, it all depends on what you're doing.

Hi Henrik,

I need to clarify my original post. The 500Kbps is the speed of the CAN network and there are about 500 messages a second. I know the 4550 can do this because the demo boards do it with no problems so it's an issue within my code. Actually, I've seen the demo boards handle 1100 messages a second. Again, I'm not sure if it's the VB code or it's the PIC code. Here is what I have for the PIC code:

START:

RxBuffer = 0                    'clear flag
 
    Low CS
       SSPBUF = %00000011                      'read command
        GoSub letclear
       SSPBUF = CANINTF            'buffer flags are contained in this mcp2515 can controller register
       GoSub letclear
        SSPBUF = 0                  'clock it out
       GoSub letclear
        RXDATA = SSPBUF             'receive buffer status
    High CS
    RxBuffer = RXDATA & %00000011   'isolate last two bits
    If RxBuffer > 0 Then   
                'a buffer is full, or 3, then both are full
        If (RxBuffer = 1) Or (RxBuffer = 3) Then
            RxBufferSub = 1
            DATA35=1                       'VB LED
            GoSub ReceiveCanSub0
        EndIf
        If (RxBuffer = 2) Or (RxBuffer = 3) Then
            RxBufferSub = 2
            DATA35=1                        'VB LED
            GoSub ReceiveCanSub1
        EndIf

GOTO START

ReceiveCanSub0:

LED_RX=0

            Low CS
            SSPBUF = 3               'read
            GoSub letclear
            SSPBUF = RXB0DLC + (RxBufferSub - 1) * 16     'receive buffer of data bytes offset 16
            GoSub letclear
            SSPBUF = 0              'dummy clock it
            GoSub letclear
            RXDATA = SSPBUF         'should contain the number of data bytes
            High CS

            DataBytesRx0 = RXDATA & %00001111  'bits 0-3 contain the number of data bytes 
            DATA1=DATABYTESRX0
            If DataBytesRx0 > 0 Then
                For i = 0 To DataBytesRx0 - 1        '(say data is 2, then 0 and 1)
                 
                    Low CS
                        SSPBUF = 3
                        GoSub letclear
                        SSPBUF = (RXB0D0 + (RxBufferSub - 1) * 16) + i
                        GoSub letclear
                        SSPBUF = 0              'dummy clock
                        GoSub letclear
                        RXDATA = SSPBUF         'mcp2515 can controller data
                    High CS

                    CanDataRx0 [i] = RXDATA
                    DATA2 = CanDataRx0[0] 
                    DATA3 = CANDATARX0[1] 
                    DATA4 = CANDATARX0[2] 
                    DATA5 = CANDATARX0[3] 
                    DATA6 = CANDATARX0[4] 
                    DATA7 = CANDATARX0[5] 
                    DATA8 = CANDATARX0[6] 
                    DATA9 = CANDATARX0[7] 
                Next
            EndIf
            Low CS
                SSPBUF = 3          'read
                GoSub letclear
                SSPBUF = RXB0SIDH + (RxBufferSub - 1) * 16  'high identifier
                GoSub letclear
                SSPBUF = 0          'clock spi
                GoSub letclear
                RXDATA = SSPBUF     'identifier
            High CS

            CanRxIDHigh0 = SSPBUF

            Low CS
                SSPBUF = 3          'read
                GoSub letclear
                SSPBUF = RXB0SIDL + (RxBufferSub - 1) * 16  'low identifier
                GoSub letclear
                SSPBUF = 0          'clock spi
                GoSub letclear
                RXDATA = SSPBUF     'identifier
            High CS

            CanRxIDLow0 = SSPBUF
       
            
            'now, clear the buffer!
            'use bit modify...
                Low CS
                        SSPBUF = %00000101              'bit modify command
                        GoSub letclear
                        SSPBUF = CANINTF                'mcp2515 can controller interrupt flags
                        GoSub letclear
                    SSPBUF = RxBufferSub                'mask for buffer number n
                        GoSub letclear
                        SSPBUF = %00000000              'clock in a clear bit
                        GoSub letclear
                High CS
                LED_RX=1
               DATA35=0                                    'VB LED 
Return

[HenrikOlsson]

Hi,

Oh, I also thought you did ADC sampling since you asked about measurment speed but OK.

I must be blind but I can't for the life of me see where in the code you're actually sending anything from the PIC to the PC.
Are you using the USART or are you using USB or am I completely missunderstanding this.

Anyway, if the PIC is executing the exact same code when you view the data "real time" as it does when you "record" it then there can't really be any problem with the PIC code or the link to the PC. The problem in that case must be with the VB code.

Why not flip an output each time the PIC processes a message and measure the frequency of that of that output. Then you'll see how many messages per second the PIC is actually processing.

/Henrik.

[Christopher4187]

Why not flip an output each time the PIC processes a message and measure the frequency of that of that output. Then you'll see how many messages per second the PIC is actually processing.

It's the VB code. I put a counter and a timestamp for each cycle through the mainloop. The results vary but the VB code is only able to poll the PIC every 10-30mS or so, and the counter jumps ~20 cycles. So in one second I can be missing a ton of packets. The VB poll looks like this:
Time (HH:MM:SS:MS) Counter(X)
11:07:25.92 / 58
11:07:25.94 / 70
11:07:25.95 / 88
11:07:25.97 / 100
11:07:25.98 / 115
11:07:26.00 / 130
11:07:26.02 / 146
11:07:26.03 / 160

When I datalog it's worse

12:48:54.84 / 252
12:48:54.88 / 30
12:48:54.91 / 64
12:48:54.94 / 90
12:48:54.97 / 126
12:48:55.00 / 156
12:48:55.03 / 186
12:48:55.06 / 216

Now I have to look at the VB code and figure that out.........