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What is cJTAG?

Published in Debug Interface 3 mins read

cJTAG (IEEE 1149.7) is an extension to the JTAG standard (IEEE 1149.1) designed primarily to reduce the number of pins required for debug and test interfaces.

At its core, cJTAG achieves this pin reduction by multiplexing the standard JTAG signals – TMS (Test Mode Select), TDI (Test Data In), and TDO (Test Data Out) – onto a single bi-directional pin. Despite using fewer physical connections, cJTAG retains all the normal JTAG debug and test functionality, making it a highly efficient interface for space-constrained devices.

Understanding the Need for cJTAG

The original JTAG standard (IEEE 1149.1), while essential for boundary-scan testing and on-chip debugging, typically requires four pins: TCK (Test Clock), TMS, TDI, and TDO, plus an optional TRST (Test Reset).

As electronic devices become smaller and more integrated, the number of external pins available for interfaces like JTAG becomes a critical constraint. This is particularly true for small microcontrollers, complex System-on-Chips (SoCs) with multiple cores, and portable or wearable electronics where package size is paramount.

cJTAG was developed to address this challenge, providing a way to maintain robust debug and test capabilities without consuming valuable pin resources.

How cJTAG Reduces Pins

The key innovation in cJTAG is the use of a multiplexed interface. Instead of separate pins for TMS, TDI, and TDO, these signals are combined onto a single pin, often referred to as the data pin.

Here's a breakdown of the pin comparison:

JTAG (IEEE 1149.1) cJTAG (IEEE 1149.7)
TCK (Clock) TCK (Clock)
TMS (Mode Select) -
TDI (Data In) -
TDO (Data Out) -
TRST (Reset, Opt.) TRST (Reset, Opt.)
Total: 4-5 pins Total: 2-3 pins

The multiplexing process uses specific protocols and timing within the cJTAG standard (IEEE 1149.7) to differentiate between mode selection data (TMS) and test data (TDI/TDO) on the shared pin.

Benefits of Using cJTAG

Implementing cJTAG offers several advantages for system designers and developers:

  • Reduced Pin Count: The most significant benefit is the reduction from 4 or 5 pins to just 2 or 3, freeing up valuable I/O pins for other purposes.
  • Smaller Package Size: Fewer pins can lead to smaller device packages, crucial for compact designs.
  • Lower Manufacturing Costs: Reducing pin count can also contribute to lower package and board manufacturing costs.
  • Maintained Functionality: Despite the reduced pin count, cJTAG retains the full capabilities of standard JTAG for tasks like:
    • Boundary Scan Testing
    • On-chip debugging
    • Flash programming
  • Compatibility: cJTAG is designed to be compatible with and build upon the existing JTAG infrastructure, allowing for relatively smooth integration into existing debug tools and workflows.

Practical Application

cJTAG is increasingly found in modern microcontrollers, digital signal processors (DSPs), and complex SoCs, particularly those targeting embedded systems, mobile devices, and the Internet of Things (IoT), where size and pin limitations are common. Debug probes and software tools designed for cJTAG understand the multiplexing scheme and can communicate with the target device using the reduced pin interface.

In essence, cJTAG provides a critical evolution of the JTAG standard, offering a pin-efficient solution for debug and test in today's highly integrated and compact electronic designs.