Impedance Matching a JTAG Emulator Probe

uh, series resistance to terminate a high Z ?? what about 100 ohm up and

Can't comment on your questions, don't understand. You can stop 'ringing' by making certain looking back up your cable is decently matched.

Can't find the URL for the online calculator, but they exist. I use two ways to calculate PCB trace widths:

1. onld DOS program called DIMSTRIP
2. free femm and do a true L/in and C/in to find the 'real' Z, more tedious but more accurate. Just guess in the range of 50 to 80 to 100 mils wide, but keep SHORT and won't matter, use 10-20 mils if less than 2 inches [from memory]. If you want a copy of DIMSTRIP let me know it's about 50KB and doesn't install, just runs. also calculates stripline and pairs of lines in case you ever need to do balanced

Their ribbon cable has TCK straddled by two grounds, so it's probably in the ballpark of 75 ohms. The other signals probably crosstalk like hell.

You might put a terminator resistor option near your CPU, on TCK. Add the resistor if you need to, 100 ohms to ground maybe. A little overshoot is a good thing. Keep the traces from the jtag to the CPU short, may as well go for 75 ohms.

We do something similar, jtag pod, ribbon cable, 10 pin jtag header near the ARM, but we just have some 10K pullups, no other termination.

Agilent's free APPCAD is an easy way to calculate trace impedances.

Yes, you should be concerned about the whole arrangement. The series resistors should be used at the source to match the driving impedance to the traces which will halve the signal voltage initially. The traces should be a constant impedance all the way to the receiver. The receiver high input impedance then reflects the entire signal which then is doubled returning to the driver. However the receiver sees both the initial signal and the reflected signal with no delay and so just sees a clean edge.

If you set your traces to half the impedance of the cable you will cause an inverted reflection at that junction in one direction and a non-inverted reflection in the other, so that is not advised.

The TDO signal should have a series resistor on the target close to the driving pin. The other signals should have series resistors in the I-jet unit. Series resistors on the target board are a compromise. They will help somewhat, but not properly.

It is very unlikely that the board traces in the I-jet are matched to the cable or that they have used appropriate termination resistors internally. So you are going to get some signal distortion no matter what you do. That's why the JTAG clock speed is often limited. If the distortion is bad enough it can cause double clocking which will give errors no matter what clock speed you use.

Den onsdag den 24. september 2014 21.07.17 UTC+2 skrev John Larkin:

I haven't measured it, but I believe standard 0.050" flat cable in a G-S-G configuration is ~100R

I can't remember ever seeing JTAG terminated other than maybe 20R series

-Lasse

We have TDR'd the fine pitch (25 mil cable pitch) ribbon cable GSG at

The 50-mil stuff differential (GSSG) is just about 100 ohms.

The beauty of putting a resistor footprint on TCK is that you can put a capacitor there if you get into big trouble.

Den onsdag den 24. september 2014 22.43.09 UTC+2 skrev John Larkin:

ineteresting, just googled and random datasheet

says 105R for G-S-G

yes, just enough lowpass to tame the edges and footprints are free

-Lasse

I checked our measurements and the 50-mil ribbon is indeed a bit over

It's the 25-mil pitch stuff that we measured in the low 70s.

I think ST have been reading the same hymn-sheet ! They have positions on the board for series, pull-up and pull down, currently populated by

Michael Kellett

Version 4 SHEET 1 1748 680 WIRE 0 0 -64 0 WIRE 992 0 928 0 WIRE -64 32 -64 0 WIRE 0 32 0 0 WIRE 928 32 928 0 WIRE 992 32 992 0 WIRE 0 144 0 112 WIRE 0 144 -272 144 WIRE 144 144 0 144 WIRE 192 144 144 144 WIRE 992 144 992 112 WIRE 992 144 720 144 WIRE 1152 144 992 144 WIRE 1280 144 1200 144 WIRE 1312 144 1280 144 WIRE 1200 160 1200 144 WIRE -272 176 -272 144 WIRE 0 176 0 144 WIRE 720 176 720 144 WIRE 992 176 992 144 WIRE 1152 176 1152 144 WIRE -272 272 -272 256 WIRE 0 272 0 256 WIRE 0 272 -272 272 WIRE 720 272 720 256 WIRE 992 272 992 256 WIRE 992 272 720 272 WIRE 1152 272 1152 224 WIRE 1152 272 992 272 WIRE 992 288 992 272 WIRE 1200 304 1200 240 WIRE 0 336 0 272 WIRE 992 400 992 368 FLAG -64 32 0 FLAG 0 336 0 FLAG 144 144 GSG FLAG 928 32 0 FLAG 992 400 0 FLAG 1200 304 0 FLAG 1280 144 GSSG SYMBOL current -272 176 R0 WINDOW 3 24 80 Invisible 2 WINDOW 123 40 60 Left 2 WINDOW 39 0 0 Left 2 WINDOW 0 41 14 Left 2 SYMATTR Value "" SYMATTR Value2 AC 1 SYMATTR InstName I1 SYMBOL res -16 160 R0 SYMATTR InstName R12 SYMATTR Value {2*Z} SYMBOL res -16 16 R0 SYMATTR InstName R23 SYMATTR Value {2*Z} SYMBOL current 720 176 R0 WINDOW 123 41 66 Left 2 WINDOW 39 0 0 Left 2 WINDOW 0 55 28 Left 2 SYMATTR Value "" SYMATTR Value2 AC 1 SYMATTR InstName I2 SYMBOL res 976 160 R0 SYMATTR InstName R_23 SYMATTR Value {2*Z} SYMBOL res 976 16 R0 SYMATTR InstName R_34 SYMATTR Value {2*Z} SYMBOL res 976 272 R0 SYMATTR InstName R_12 SYMATTR Value {2*Z} SYMBOL e 1200 144 R0 SYMATTR InstName E SYMATTR Value 1 TEXT 384 240 Left 2 !.param Z=75 TEXT 352 144 Left 2 !.ac dec 100 1 1k TEXT -208 -88 Left 3 ;GND-SIGNAL-GND TEXT 840 -80 Left 3 ;GND-SIGNAL-SIGNAL-GND TEXT -176 -144 Left 3 ;Single Ended TEXT 944 -136 Left 3 ;Balanced TEXT 160 56 Left 2 ;Zsingle = V(GSG)/1A TEXT 1288 72 Left 2 ;Zbalanced = V(GSSG)/1A TEXT 40 -240 Left 5 ;Impedances of Belden Ribbon Cable TEXT 128 320 Left 4 ;Zsingle = 75 ohm TEXT 1288 328 Left 4 ;Zbalanced = 100 ohm