ARM Technical Support Knowledge Articles

Rear panel JTAG port does not work on MPS

Applies to: MPS


The MPS box has a 20-way JTAG connector on the back panel for connecting a debug ICE unit. It also has an internal 38-way Mictor socket for combined ICE/Trace connection. The MPS automatically selects which JTAG connector to use, depending on the state of pin 20 of the rear panel JTAG header. Depending on the pinout of your JTAG ICE connector, this selection may not work properly.


All of the even-numbered pins on ARM's standard 20-way JTAG header (with the exception of pin 2, which is VSupply) are supposed to be connected to GND. On a piece of 20-way ribbon cable, this gives good noise immunity since signal wires are interleaved with ground wires. ARM's RealView ICE and older Multi-ICE JTAG units both have these multiple ground connections. It is not strictly necessary to connect all of the ground wires just to make the interface work however. Some JTAG ICE manufacturers elect to leave some or all of the redundant ground wires unconnected.

The MPS design assumes that pin 20 of the JTAG cable is grounded by the ICE unit and it is this connection that instructs the internal CPLD to source the JTAG signals from the back panel instead of the internal Mictor. If pin 20 is left unconnected, the internal Mictor JTAG port will be activated and the rear panel JTAG port will be inoperative.


The only way to enable the rear panel JTAG port is to ground (or pull to logic 0) pin 20 on the JTAG DIL header. Pin 20 is pulled up to 3.3V inside the MPS box by a 10k resistor. Thus it is not necessary to short pin 20 to ground on the external JTAG header; it merely needs to be pulled low enough with a suitable resistor to bring the voltage on that pin well below the 0/1 threshold for LVTTL. 3.3k Ohms or less should work.

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