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http://sf.net/apps/mediawiki/urjtag/index.php?title=Cable_Altera_USB-Blaster

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Altera USB-Blaster

------------------

General

-------

        _________

       |         |

       | AT93C46 |

       |_________|

        __|__________    _________

       |             |  |         |

  USB__| FTDI 245BM  |__| EPM7064 |__JTAG (B_TDO,B_TDI,B_TMS,B_TCK)

       |_____________|  |_________|

        __|__________    _|___________

       |             |  |             |

       | 6 MHz XTAL  |  | 24 MHz Osc. |

       |_____________|  |_____________|

Quoting from ixo.de (http://www.ixo.de/info/usb_jtag/)

usb_jtag/device/c51/usbjtag.c comments:

usb_jtag firmware now happens to behave just like the combination of

FT245BM and Altera-programmed EPM7064 CPLD in Altera's USB-Blaster.

The CPLD knows two major modes: Bit banging mode and Byte shift mode.

It starts in Bit banging mode. While bytes are received from the host

on EP2OUT, each byte B of them is processed as follows:

Bit banging mode

----------------

1. Remember bit 6 (0x40) in B as the "Read bit".

2. If bit 7 (0x80) is set, switch to Byte shift mode for the coming X

   bytes ( X := B & 0x3F ), and don't do anything else now.

3. Otherwise, set the JTAG signals as follows:

   - TCK/DCLK high if bit 0 was set (0x01), otherwise low

   - TMS/nCONFIG high if bit 1 was set (0x02), otherwise low

   - nCE high if bit 2 was set (0x04), otherwise low

   - nCS high if bit 3 was set (0x08), otherwise low

   - TDI/ASDI/DATAO high if bit 4 was set (0x10), otherwise low

   - Output Enable/LED active if bit 5 was set (0x20), otherwise low

4. If "Read bit" (0x40) was set, record the state of TDO(CONF_DONE) and

   DATAOUT/(nSTATUS) pins and put is as a byte( (DATAOUT<<1)|TDO) in the

   output FIFO _to_ the host.

Byte shift mode

---------------

1. Load shift register with byte from host

2. Do 8 times (i.e. for each bit of the byte; implemented in shift.a51)

   - if nCS=1, set carry bit from TDO, else set carry bit from DATAOUT

     (Active Serial mode)

   - Rotate shift register through carry bit

   - TDI := Carry bit

   - Raise TCK, then lower TCK.

3. If "Read bit" was set when switching into byte shift mode, record the

   shift register content and put it into the FIFO to the host.