Circuit and method of JTAG testing multichip modules

摘要

A multichip module (10) having one or more IC die (12-18) supports JTAG testing with a plurality of registers (20-26) within each IC die. JTAG testing requires a one cycle delay bypass mode where registers within an IC not under test are bypassed. To support bypass mode when JTAG testing the multichip module on a printed circuit board, a bypass circuit around the multichip module provides the one cycle delay. The bypass circuit monitors the test data signals to the multichip module and enters bypass mode upon detecting a predetermined sequence of logic states during the instruction sequence. Otherwise, the test data signal passes through a plurality of registers within each IC die. The detection may be performed by counting logic states or otherwise monitoring in the instruction sequence.