how to test your jtag

[EVD]

There are times when we need to test the Jtag hardware to be sure that it is functioning properly. Jkeys 2.0.1 or higher has a diagnostic feature built in that lets you test each of the signal wires individually.

You will need a Multimeter set to VDC. Micro clips are also a benefit, as you will have a difficult time holding the leads and testing the appropriate signal. If leads are not available then get a friend to hold them for you during your testing.

Plug your Jtag interface into your printer port and then open the Jkeys 2.0.1 program and click on "TOOLS" located on the top bar and then "Port Diagnostics" (you will not have your ird plugged into your Jtag for this test.)

If your Jtag uses external power then turn that on as well.

You will see this screen:


We will be testing TMS, TCK, TDI, TRST and TDO (TDO is located in the Status Bits section). If you have a simple Jtag interface then you may only have a reference to the DB25 pins 2,3,4,5 and 13.

Turn your Meter to the VDC (volts DC) setting.


Clip or hold the BLACK (ground) lead to the Ground lead of your Jtag hardware. This is the lead that is connected to pins 18 through 25 of your DB25.

Then hold or clip the RED (positive) lead to the lead marked TMS or the resistor connected to DB25 pin 2.

Now using your mouse, click on the TMS box so there is a check mark in the box and observe your meter.

You should see close to the following:


It may be normal to see anything from 4.5v and up to 5v. Click on the TMS box again and you should see it fall from your current reading to 0v or the following:


You may see a few millivolts depending on the accuracy of your meter but it should not be more than half a volt.

Click on and off TMS several times to observe the signal go high and low. If yours does not toggle then there could be several reasons for this.

1) Your on the wrong pin.
2) Ground is not correct.
3) Wrong setting on your meter.
4) Your Jtag is broken.
5) Your computer printer port may be broken.
6) Your printer cable may not be a 1 to 1 but is a null modem cable.
7) Some other reason.

We will assume at this point that TMS did work. Move the RED lead to the next signal TCK and follow the same procedure as above.

Do the same for TDI and TRST.

The last Signal to test is TD0; this signal is read from the status box of the diagnostic port. Since this signal is at a low voltage when not connected to anything the TD0 box will be blank. We need to force TDO high (apply 5v to it) so that we can see it go high when we read the status port.

To do that we click on TMS and using a jumper we clip TMS to TDO, then click on read status.


Notice that the TDO (DB25 pin 13) now has a check mark in the Status Bits in. Click on TMS to turn that bit off and do a read status again and you’ll see the TDO bit check mark is gone.

If all the above tests have passed then your Jtag is operational. If you have a problem with a signal or two then check for continuity in your wires or shorts such as solder bridges in your work.

If checking for continuity then you should unplug your Jtag and remove any power from it.

Put your meter on ohms:


With the black and red leads apart your meter should show nothing like this:


When you touch the red and black leads together you should see continuity or commonly called a short:


Touch your leads together and see what results your meter shows. Do it several times so you have a good idea what it looks like when the leads are not touching, and when they do touch.

To see if a wire has continuity then place one probe on one end of the wire and then the other probe on the other end of the wire. If the wire is not broken and has continuity then you should see a short on the meter (like when you touched the two leads together) or as an open (as when the leads were not touching each other).

If there is a resistor in between the end of the wire and say a connector for example you should see that indicated on the meter as well. For example if you have a 100-ohm resistor then you should see pretty close to 100 ohms on the meter.

You should not see much higher or lower than 5 to 10%. If you do then perhaps there is a short somewhere or the wrong resistor was used.