How are JTAG functions implemented on SCT multichip modules such as the DS21Q352/DS21Q55?
A4. A multichip module is any device that contains multiple die or devices housed in the same package. The DS21Q352, DS21Q354, DS21Q552, DS21Q554, and DS21Q55 are all examples of quad-port, SCT multichip modules. Each of these multichip modules has four single-port devices with daisy-chained JTAG functionality. This is necessary because each single-port device contains a JTAG test access-port controller, and because the BSDL language lacks the ability to define multiple test access-port controllers. In the module, some pins such as address and data buses are tied together. The JTAG pins are either tied in parallel or daisy-chained, as appropriate. More detail can be found in the device’s data sheets. To create the proper JTAG scan chain, four copies of the single-port version of the device need to be placed in the JTAG chain. For example, if a DS21Q55 were placed on the board, the proper BSDL file would be the DS2155. When using multichip modules in the JTAG chain, the design may need
