ArduinoISP on the Leonardo

A Leonardo programming an attiny2313

A while ago I noticed this thread about burning an Atmega 328 bootloader with an Arduino Leonardo. It turned out that the ArduinoISP sketch that comes with the arduino ide (version 1.0.1) does not work out of the box on the recent Leonardo model. I decided to buy one and help investigating the problems reported in that thread. Following issues were posted:

• Issue 998: Leonardo serial buffer overrun when sending more than 64B (from pc to leonardo).
• Issue 995: Unintentional autoreset after uploading a sketch to a leonardo

In mean time fixes for these issues are committed in the development tree so I expect them to become available in an upcoming release of the ide. Untill then, here is a wrap up on how to use the leonardo as an isp programmer with Arduino 1.0.1. Edit: the fix proposed below in step 3 made it into Arduino 1.0.2, so it is no longer needed. The rest is still applicable.

1. Like in the picture above, connect the target’s MISO/MOSI and SCK signals to the ICSP header of the Leonardo. Don’t connect them to digital pins 11, 12 and 13: on the leo there is no SPI on these pins.

Edit: the picture above is just for setting the scene. Ja450n (thank you) sent me a nice Fritzing drawing, featuring an attiny85,  that is more illustrative:

2. In ArduinoISP.ino, change this line:
#define RESET SS
into:
#define RESET 10

Edit November 2015: Since Arduino 1.6.6 this change is already in the IDE.

This states we want to use pin 10, to reset the target mcu .(the brown wire in the picture).
On the leo, SS is not available on arduino pin 10. Actually it is not available on any arduino pin, the atmega pin that exposes SS is used to drive the RX led. Fortunately it is perfectly ok to use pin 10 (or any other digital pin) to drive the target’s reset. (The only requirement for the SS pin is that it is configured as an output (which makes the leo an SPI master). This requirement is fulfilled since it drives the rx led.

3. Locate the CDC.cpp and USBCore.cpp files in your arduino installation directory and apply following modifications (you may want to make a copy first):
In CDC.cpp, the accept routine should look like this:

void Serial_::accept(void)
{
    ring_buffer *buffer = &cdc_rx_buffer;

    int i = (unsigned int)(buffer->head+1) % SERIAL_BUFFER_SIZE;

    // if we should be storing the received character into the location
    // just before the tail (meaning that the head would advance to the
    // current location of the tail), we're about to overflow the buffer
    // and so we don't write the character or advance the head.

    // while we have room to store a byte
    while (i != buffer->tail) {
        int c = USB_Recv(CDC_RX);
        if (c == -1)
            break; // no more data, we're done
        buffer->buffer[buffer->head] = c;
        buffer->head = i;

        i = (unsigned int)(buffer->head+1) % SERIAL_BUFFER_SIZE;
    }
}

In USBCore.cpp, search following snippet:

#ifdef CDC_ENABLED
    USB_Flush(CDC_TX); // Send a tx frame if found
    while (USB_Available(CDC_RX)) // Handle received bytes (if any)
        Serial.accept();
#endif

and change it into:

#ifdef CDC_ENABLED
    USB_Flush(CDC_TX); // Send a tx frame if found
    if (USB_Available(CDC_RX)) // Handle received bytes (if any)
        Serial.accept();
#endif

4. Compile and upload ArduinoISP to the leo.

5. In the ide, open the serial monitor and see to it that the baud rate is not equal to 1200bps. Close the serial monitor again. This step removes the magic baud rate from the serial port which would cause unintentional auto resets.

6. On linux (and probably mac, though I don’t have a mac to try it) you can just go to the “Tools > Programmer” menu and select “Arduino as ISP” as programmer, Now the leo is ready to act as an isp. From here, follow the regular procedure to burn a bootloader in the target. Or you can use avrdude from the command line to burn any hex file in the target…

7. On windows an extra step is required. When selecting “Arduino as ISP” as programmer, the IDE will invoke avrdude and instruct it to use the stk500v1 protocol. Currently (arduino 1.0.2 and 1.0.3) this does not work on windows. It will result in following error message:

avrdude.exe: Send: 0 [30]   [20]
avrdude.exe: Send: 0 [30]   [20]
avrdude.exe: Send: 0 [30]   [20]
avrdude.exe: Recv:
avrdude.exe: stk500_getsync(): not in sync: resp=0x00

Note that avrdude sends out stuff, but does not receive anything. The leo’s rx led will flash but the tx led stays dark.

As a  temporary work around it was suggested (see here) to define a programmer that instructs avrdude to use the “arduino” protocol instead of “stk500v1”. To do so, create a sub directory named “hardware/leofix” in your sketchbook directory. In that directory install a file “programmers.txt” with following content:

arduinoispleo.name=Arduino as ISP (Leonardo)
arduinoispleo.communication=serial
arduinoispleo.protocol=arduino
arduinoispleo.speed=19200

After restarting the IDE, you will have an entry “Arduino as ISP (Leonardo)” in the “Tools > Programmers” menu. This will do the job on windows too.

The reason stk500v1 does not work is that upon opening the virtual com port, windows/avrdude do not request to assert the “virtual” DTR and RTS signals. The leonardo refuses to send out data under these conditions so it never writes data back to avrdude. When using the arduino protocol, before starting programming, avrdude will briefly deassert DTR and RTS and assert them again. This would trigger autoreset if you were uploading a sketch to an arduino bootloader. But in this work around, we use the side effect that this leaves DTR/RTS asserted during programming which makes the leonardo talk..

I filed an issue for this too:

• Issue 1182: ArduinoISP on the Leonardo does not work on windows.

8. Another reason why ArduinoISP may not work, may come from the fact that the Leonardo is a composite device (it combines a virtual comm port and a HID (keyboard/mouse) in one USB device). Obviously, if you are on an old operating system that does not support composite devices, ArduinoISP will not work, nor will any sketch that uses USB. (Confusingly, uploading sketches is possible because the boot loader is not a composite device.)

Windows has support from XP SP3 on and from Vista SP1 on, Mac has support since Lion, see here for more background.

Since ArduinoISP does not need the HID stuff, a work around is to build it as a “serial port only” sketch. This is described here.