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JTAG - I2C Arbitration Disclosure Number: IPCOM000012153D
Original Publication Date: 2003-Apr-14
Included in the Prior Art Database: 2003-Apr-14
Document File: 2 page(s) / 28K

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JTAG and I2C Arbitration

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JTAG - I2C Arbitration

1. Description

JTAG (IEEE 1149.1) is the current industry standard test access port (TAP) for integrated circuits. With applications where another interface is desired, it makes sense to translate this into a JTAG instruction stream to get by with one single bus interface on the chip. In the specific case where this invention was developed, this second interface is an I2C (Integrated Circuit Interconnect, Philips Corp, public domain). The problem arises when both interfaces are used for the same purpose
(e.g.chain scanning) at the same time. Both the native JTAG and the I2C-generated JTAG commands cannot be handled concurrently, thus some form of arbitration is necessary.

2. Solution

    This publication offers a hardware assisted semaphore arbitration. The semaphore is provided by hardware as an I2C/¬JTAG pin on the chip. That way the native JTAG and the I2C device just need to observe this pin to determine if they have access to the chip or not. The setting of this pin is done by writing to a register on the chip. To avoid the setting of the I2C/¬JTAG pin before the current JTAG operation that writes this register has finished, the pin is only changed during the JTAG states Test-Logic-Reset or Run-Test-Idle. Thus it is impossible for the two JTAG instruction streams to overlap, provided they observe the semaphore.

    This semaphore solution requires that the device that owns the semaphore gives it back to the other device after it is done, and v...