I'm Learning about JTAG ....

And I'm finding it very similar to SPI, 1 pin to signal the device you wish to use by sending it low/high, 1 pin for send 1 for recv...

So i'm wondering if SPI is based (however loosely) on this JTAG method from the 80's..

No, I don't think so. SPI predates JTAG. An both of them are little bits of formal definition on top of "obvious" digital electronics concepts like latches and shift-registers that go "way back."

Prior art includes uarts, synchronous serial communications standards, the "centronics parallel printer interface", BBN-1822 "Specifications for the interconnection of a host and a IMP", and a bunch of other stuff (shift registers were used for data storage before RAM had been invented, for example.)

Weren't the originals 12AU7 tubes setup as flip-flops?... With NE2's as indicators?

Doc

...that is probably one of the best talks/explanations on JTAG as a whole, in a language which clearly dosen't sound like his first!

Hi,
I'm posting here, just to ask a question about the jtag.
I'm a beginner and I'm trying to use the mkii mc that is an anofficial JTAG for avr system.
My question is: When I connect the JTAG with the board, must the board be connected with the pc too?

Do you mean the AVR ISP MKii? The in-system programmer for the 8-bit AVR devices?

If so, yes the board needs to be powered - but it is not a JTAG device, it programs the chip using an SPI interface.

DarkCoffee:
My question is: When I connect the JTAG with the board, must the board be connected with the pc too?

If you mean an Arduino-style board (as this is after all, an Arduino forum) which has a USB connection to the PC but you propose to program it through a USBASP adaptor, then you would only connect one or the other. The USBASP provides power whilst programming through the six pin connector.

It is an interesting detail that on eBay, most USBASP programming adaptors are sold with the ten pin connector (of which only six pins are actually used) and the ten-to-six pin adaptors are sold separately for the same price as the programmer itself. Apparently, the primary market for these is not the Arduino user community as such, but the "multicopter" enthusiasts - which I have something of a mind to get into at some stage. 8)

This one: http://www.atmel.com/tools/AVRJTAGICEMKII.aspx ?
Presumably with an Arduino MEGA ? (The ATmega328 doesn’t have any jtag.)
You shouldn’t need the Arduino’s normal PC connection when using that, but it shouldn’t hurt anything either. The normal MEGA USB connection only talks to the Arduino CPU via the UART interface.