ASM51

(tópico que postei no AVRfreaks)

I present a firmware patch for the Lcsoft USBASP V2.0, a cheap and very common AVR programmer from ebay:



When using the slow SCK option (jumper J3), the connection from AVRdude to the target AVR may be unstable:
- sometimes the signature is not recognized...
- sometimes the program verification fails somewhere...
- etc.

With a logic analyzer, I discovered the MOSI and SCK pins are NOT following the serial programming datasheet specification, which uses CPOL=0 and CPHA=0. The USBasp is actually clocking the MOSI line at the SCK rising-edge.

I reverse engineered the chinese programmer and found the bug. The poor code does SPI emulation with the following pseudo-code:

Código: Selecionar todos

spi_byte_exchange(byte c)
{
  i = x = 0;

  repeat
  {
    MOSI = c.bit7;        // output
    x = (x << 1) | MISO;  // sampling
     
    SCK=1;  // rising-edge
    wait();

    SCK=0;  // falling-edge
    wait();

    c <<= 1;

    if ( ++i == 8 )
       return x;
  }
}


This patch reorder things, resulting in the pseudo-code:

Código: Selecionar todos

spi_byte_exchange(byte c)
{
  i = x = 0;

  repeat
  {
    MOSI = c.bit7;        // output
     
    wait();
    SCK=1;  // rising-edge
    x = (x << 1) | MISO;  // sampling

    wait();
    SCK=0;  // falling-edge

    c <<= 1;

    if ( ++i == 8 )
       return x;
  }
}


The digital waveforms below shows the effect of the patch on the MOSI line:



The 0xAC and 0x53 bytes is the beginning of the Programming Enable command.

The patched firmware is available from here: (11KB)
https://docs.google.com/file/d/0By5yf3g1ZJa9UnNLWWcwUXNWaWM/edit?usp=sharing

You need another AVR programmer to update your Lcsoft USBASP. This can easily be done with another USBASP with the following command-line:

Código: Selecionar todos

avrdude -p m88p -P usb -c usbasp -U flash:w:USBasp_lcsoft_patched.hex


Hope this patch helps to solve some of the painful instability issues with this programmer.

Note 1: I've tried to reflash using the 2009 and 2011 firmwares from www.fischl.de/usbasp, and I got the usbasp not recognized as an USB device.
Note 2: Tested with atmega88 devices. Use it at your own risk.