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Visitor jaguila
Visitor
3,797 Views
Registered: ‎09-30-2014

Spartan 6 TCK and TMS routing

Hi,

 

I'm developing a board based on two Spartan 6 FPGAs and I have some doubts about TCK and TMS routing. As both TCK and TMS have to be connected to two FPGAs, I can't route them with 50 ohms impedance unless I route from JTAG connector to first FPGA TCK/TMS pin and from first FPGA TCK/TMS pin to second FPGA TCK/TMS pin.

 

Page 58 in ug380 indicates that "TCK should transition monotonically at all receivers to ensure proper JTAG functionality and must be properly terminated. The quality of TCK can limit the maximum frequency for reliable JTAG configuration".

 

In order to get a TCK/TMS monotical transition, I suppose that the routing distance from TCK/TMS JTAG connector pins to FPGAs pins should be similar, and this is not possible with 50 ohms routing, because I would have an impedance discontinuity due to the Y routing method.

 

What's the recommendations for this routing?, do I have to put a termination resitor at the end of the TCK/TMS line transmission?

 

Thank you,

 

Jaime,

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2 Replies
Teacher eteam00
Teacher
3,770 Views
Registered: ‎07-21-2009

Re: Spartan 6 TCK and TMS routing

... As both TCK and TMS have to be connected to two FPGAs, I can't route them with 50 ohms impedance unless I route from JTAG connector to first FPGA TCK/TMS pin and from first FPGA TCK/TMS pin to second FPGA TCK/TMS pin.

 

The routing topology you describe is called 'daisy-chain'.  This is perfectly acceptable, and is commonly used practice.

 

Page 58 in ug380 indicates that "TCK should transition monotonically at all receivers to ensure proper JTAG functionality and must be properly terminated. The quality of TCK can limit the maximum frequency for reliable JTAG configuration".

 

In order to get a TCK/TMS monotical transition, I suppose that the routing distance from TCK/TMS JTAG connector pins to FPGAs pins should be similar,

 

No, you are mistaken.  'Monotonic transitions' is another way of saying 'no ringing on rising and falling edges'.

 

and this is not possible with 50 ohms routing,

 

This is also incorrect.  Equidistant routing is not required, and 50 ohm trace impedance does not have any bearing on trace length (in this context).

 

because I would have an impedance discontinuity due to the Y routing method.

 

You should not be using Y (or radial) trace layout for TCK (or any clock signal, generally).  You should be using 'daisy-chain' routing.

 

What's the recommendations for this routing?, do I have to put a termination resitor at the end of the TCK/TMS line transmission?

 

Use daisy-chain routing of the TCK net, and place the termination resistor at the end of the trace furthest from the JTAG connector or driver.

 

Suggest you spend some time with an experienced circuit board designer (schematic designer, not layout designer), and learn about signal integrity concepts and practice.  There are textbooks on this subject, but a real live person to explain, teach, and demonstrate is a big help.

 

If or when you find a proper teacher, ask the teacher why monotonic edges are critically important for the TCK clock signal but NOT for the TMS signal.

 

-- Bob Elkind

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Visitor jaguila
Visitor
3,758 Views
Registered: ‎09-30-2014

Re: Spartan 6 TCK and TMS routing

Thank you for your fast answer,

 

I hadn`t understood properly the word "monotonical". You have explained cleairly what I have to do. Anyway, I'll get in touch with an expert hardware designer in my company in order to learn more about this.

 

Thank you again,

 

Regards, 

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