JTAG ICE clone buffer IC question

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Good day!

I've assembled JTAG ICE clone from this schematics:
http://www.scienceprog.com/wp-co...
(I'm aware of it's limitations: no AVRStudio 5 support and limit number of MCU with JTAG).

My JTAG ICE is always powered from USB via FT232RL board I've already have. I'd like to add jumper to be able to power target from JTAG ICE (like usbtinyISP).

Additionally I want to put 74HC244 buffer (http://eldigi.ru/site/programmators/download/74HC244.pdf) on JTAG lines. I've heard that JTAG is picky about line driving characteristics. What I should consider while adding this buffer? Do I really need it for 3.3V targets?

Thanks!

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Quote:
Additionally I want to put 74HC244 buffer
Why do you want to do that? I have had a home made JTAG Mk1 for years without problems.

John Samperi

Ampertronics Pty. Ltd.

https://www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Quote:
Do I really need it for 3.3V targets?

I do not think this buffer is necessary for 1,8V targets, not mentioning 3,3V ones.

Just check the logic levels thresholds of your target and m16, especially TDO line (connected to MISO). You can use a regular voltage divider for this connection.
And do not violate Absolute Maximal Rating table parameters, like applying more than Vcc+0,5V on any pin. Internal clamping diodes withstand about 1mA so limit the applied currents some way.

No RSTDISBL, no fun!

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Ok, if my JTAG ICE is powered from 5V and target from 3.3V it seems that logic levels from JTAG will exceed Vcc+0.5V for target?

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My JTAG Mk1 get it's power from the target, so if it's 3.3V then it runs at 3.3V, if it's 5V it runs from 5V. No problems.

Edit I just looked back at the diagram and it ALSO gets it's power from the target via pin 7. The only issues you may have is that the MAX232 will not work at 3.3V, so you may need to change it for something like a MAX3232 (??) which runs from 3.3V to 5V.

John Samperi

Ampertronics Pty. Ltd.

https://www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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js wrote:
My JTAG Mk1 get it's power from the target, so if it's 3.3V then it runs at 3.3V, if it's 5V it runs from 5V. No problems.

Edit I just looked back at the diagram and it ALSO gets it's power from the target via pin 7. The only issues you may have is that the MAX232 will not work at 3.3V, so you may need to change it for something like a MAX3232 (??) which runs from 3.3V to 5V.


I do not use TTL2RS232, I use this USB2TTL instead:

Quote:

My JTAG ICE is always powered from USB via FT232RL board I've already have. I'd like to add jumper to be able to power target from JTAG ICE (like usbtinyISP).

That's why I'm thinking about buffers.

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Quote:
I do not use TTL2RS232, I use this USB2TTL instead:
So you can still have the USB chip being supplied by the 5V off the computer but the TTL side from the target at whatever the target voltage is.

John Samperi

Ampertronics Pty. Ltd.

https://www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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js wrote:
Quote:
I do not use TTL2RS232, I use this USB2TTL instead:
So you can still have the USB chip being supplied by the 5V off the computer but the TTL side from the target at whatever the target voltage is.

Thanks, the TTL levels are controlled by the VCio pin in case of FT232RL.