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Re: [avr-chat] Dragon: mega slow at dumping Flash, normal ?

From: Joerg Wunsch
Subject: Re: [avr-chat] Dragon: mega slow at dumping Flash, normal ?
Date: Sat, 4 Jul 2009 09:43:55 +0200 (MET DST)

Vincent Trouilliez <address@hidden> wrote:

>> Apart from that, JTAG is pretty fast.

> I remember that you told me (a year ago maybe..) not to expect JTAG
> to be very fast because its protocol included many delays which
> slowed things down.

You're perhaps confusing that with debugWIRE.  As debugWIRE is a
one-wire protocol, it has to live with delays, caused by RC time
constants of the circuitry.

> That said, ISP does have a clock signal too ! So fundamentally, I
> don't understand why ISP requires the target to also have its clock
> working ?

ISP is basically the SPI engine of the controller, running in slave
mode.  As the entire SPI engine is a synchronous system clocked by the
IO clock of the AVR, the IO clock must be quite a bit faster than the
ISP clock (the ISP clock line itself is sampled within the SPI engine,
and ISP clock edges are detected during that sampling).

In contrast, JTAG (outside debug mode) is also a synchronous engine,
but it is completely clocked by the JTAG master.

> Hmm, I don't think I will leave my Dragon to go back to a pure ISP
> programmer.

As long as you've got JTAG-capable controllers, sure.  However, once
you're using a smaller controller that only offers ISP, you have to
use that.

Even though debugWIRE has its deficiencies (that are mostly due to its
1-wire bus technology), once you're faced with it, it's still much
better than not being able to debug at all.

cheers, J"org               .-.-.   --... ...--   -.. .  DL8DTL

http://www.sax.de/~joerg/                        NIC: JW11-RIPE
Never trust an operating system you don't have sources for. ;-)

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