pyb.USB_VCP() performance?

[bibile]

Hi,

I run into a small performance issue.
I read an encoder and send a torque value (DAC) to a DC motor.

My control loop runs at 200Hz, using a Timer.
Everything works well, except when I try to send some data to the PC.

I use USB_VCP

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u = pyb.USB_VCP()

And write to the PC at 100Hz (another Timer) with:

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u.write(str(nbtop))

I tryed many things, using several timers, or other way, in the mainloop.
But as soon as I use the u.write(), it damages my control loop (encoder=>torque value), it still works, but less good.
I guess that u.write() uses to much time.

Is there a way to improve it?

[bibile]

Hi,

Would it be more efficient to use pyb.UART?
Actually, I will have to use a RS232/USB adapter, but why not!

Any idea?

[dhylands]

The way USB works is that it's polled by the host.

So when you want to write data to the host, the pyboard has to wait for the host to ask it if it has any data available.

I haven't looked at the code to see how much data gets buffered and when exactly it gets buffered, but part of the performance equation depends on the host and how frequently it polls.

The data is actually transferred at 12 Mbits/sec. For UART the amount of time taken (on the pyboard) will be directly proportional to the baud rate. If you set the baud rate to 115200 bits/sec, then you will get about 11520 bytes per second sent thru the UART. Which means that each character will take approx 100 usec. So you multiply the number of characters you're sending by 100 usec and that''s how long each write call will take.

[bibile]

Thanks!

I guess that with 9600, I am short.

On PC side, I simply run:

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import serial
ser = serial.Serial('COM5', 9600)
while True:
    nb = ser.in_waiting
    if nb > 0:
        buf = ser.read(nb)
        print buf

I will upgrade to 115200, that should help.

[dhylands]

Note that the baud rate on the USB serial link is totally ignored and has no bearing on any performance (at least when talking via USB to say a pyboard).

Anytime pyb.USB_VCP is involved (on the MicroPython side) then the baud rate from the host is totally ignored. The baud calculations I did in my previous message apply to using a UART.

[marfis]

Measured USB VCP performance is about 8Mbit/sec (but this can be less if the host handles several devices).

The pyboard uses the USB SOF (start of frame) event sent by the host. This fires an interrupt every 1msec. In the irq handler it checks if something is pending to send. If so it sends the data in 256 Bytes chunks every 250usec.

As @dhylands pointed out, the specified baudrate on the PC is completly irrelevant for USB VCP.

The best way to proceed for you is to toggle a pin at the start/end of your control loop and write function, so you'll get an idea on how much processing power is left to submit the values. Since you are calling the control loop in 5msec interval (and I guess you'll calculate staff there) it seems plausible that your board is already close to 100% busy.

You'll need a small logic analyser for that, but it's well invested money.