Updated 18 June 2003 -- Maintained by E. Hazen


The ATLAS CSM Mini-Adapter provides an interface between the CSM-0 VME module and up to 18 production-style mezzanine boards. In normal operation JTAG access is provided to all connected mezzanine boards in a chain. Unconnected mezzanine boards are bypassed by simple on-board logic on the adapter. In case of mezzanine board or cable problems, the JTAG chain may not work, and can be hard to diagnose.

This page describes a mechanism by which individual mezzanine boards can be switched in and out of the JTAG chain under software control.

Diagram of mini-adapter logic
Block Diagram of Mini-Adapter Logic

Implementation of this logic is limited by two "features" of the PCB:
  1. There is no power-up reset logic on the board
  2. There are no spare signals allocated for communication amongst the 4 CPLDs. Keeping in mind these limitations, we describe here one possible implementation of special firmware for JTAG control of the mini-adapter.

JTAG Programming

At power-up the adapter is in an undefined state. Execute the following sequence of operations (4) times to reset it: At this point the length of the instruction register may be measured if desired by entering the Shift-IR state and writing a recognizable pattern, and waiting to see if the pattern is returned after repeated shifting. Before exiting via Update-IR, one must shift (4) time the MEZZCTRL instruction.

To read the state of the mezz SENSE inputs and enable a specific set of mezz card ports, perform the following sequence of operations:

There are two unused bits in the mask register, positions 14 and 19. The first bit shifted out is bit 19; the last bit shifted is bit 0;

At this point the JTAG chain will provide access only to the selected mezz cards. If a port with a non-working mezz card is selected, no data will be returned. One must begin again with the reset sequence at the top to regain control of the adapter. The adapter will not appear as a device in the JTAG chain unless the instruction code B"10101" is entered in the Shift-IR state as described above.


jtag_primer.pdf JTAG Testability Primer from T.I.
A JTAG Introduction