debugWire and RESET

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The JTAGICE MKii Quick Start Guide states "Pull-up lines on the RESET line must not be smaller than 10K and there should be no capacitive load."

But the Butterfly schematic shows a 10nf cap from RESET to GND. What's the scoop?

Also, is it correct that ISP is a programmer only? No debug capability? (I know, ISP = In System Programmer, but just checking.)

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The Butterfly processor is a JTAG ATmega169V. There is no debugWIRE involved. Read this document for more answers:

AVR042: AVR Hardware Design Considerations:
http://www.atmel.com/dyn/resourc...

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OK, bad example. I am actually using attiny2313 (mentioned Butterfly as example). According to the JTAGICE MKii Quick Start Guide there cannot be a cap on RESET. Therefore no external reset. Is this correct?

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Quote:

Therefore no external reset. Is this correct?

Yes.

This is just while debugging using debugWire though - for your deployed design you need to take your normal reset precautions and if there's a chance the unit may need a dW attachment in the field include jumpers so that the pin can be isolated again.

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Thanks, Clawson. That's what I thought. Just verifying before I modify artwork for client's 3 PCBs.

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aero1 wrote:
According to the JTAGICE MKii Quick Start Guide there cannot be a cap on RESET. Therefore no external reset. Is this correct?
You obviously did not read or at least understand AVR042. The external capacitor is not responsible for any AVR external reset function. The cap is in fact only for added noise filtering to help prevent any noise induced unwanted external reset. The AVR has an internal power on reset circuit and does not use the external reset pin for power up reset (for some examples see the tiny2313 MCUSR register EXTRF and PORF bits, also Figure 14 Reset Logic). It is mostly ancient microprocessor architectures that required a resistor and cap RC circuit on the reset pin for power up reset. There is in fact an external reset when the AVR reset pin is pulled low and you are not in debugWire mode (assuming you do not program the RSTDISBL fuse). In debugWIRE mode the reset line function is changed and the noise filter cap interferes with the debugWIRE data.
AVR042 wrote:
To protect the RESET line further from noise, it is an advantage to connect a capacitor from the RESET pin to ground. This is not directly required since the AVR internally have a low-pass filter to eliminate spikes and noise that could cause reset. Applying an extra capacitor is thus an additional protection. However, note that this capacitor cannot be present if debugWIRE is used.