We will in this article take a closer look at the JTAG interface and how Fuses and Lockbits affect operation. If you are using, or planning to use the JTAG interface, you should definitely spend a couple of minutes reading this article!
Introduction
New megaAVR devices with more than 8KB of Flash, include a JTAG interface for Programming, Boundary Scan and On-chip Debugging. This article will take a peek at some of the features that are somewhat hidden, or difficult to find, in the datasheets.
JTAG Fuses and Security
The first source of confusion when looking at the JTAG interface is understanding the fuses, and how they affect the behavior of the device and/or JTAG interface. In addition; An IO control bit “JTD” is a available. Unlike the fuses this bit can be set at run-time allowing you to disable the JTAG / OCD functionality in software during program execution. So what is the difference between the JTAGEN and OCDEN fuse? What do they do, and what is the consequence of only programming one of them?
To save you some time fine-reading the Datasheet, we here at AVRfreaks have compiled the following table and functional block diagram that should explain the relationship between these fuses and functionality. The block diagram below the table shows the relationship between the Fuses and JTD control bit. Note that setting either of the Lock bits LB1or LB2 will disable OCD.
table shows the relationship between the Fuses and
As shown in the figure and table, setting the correct fuses and keeping the device unlocked is essential to be able to access the on-chip OCD system. As shown, setting lock bits will disable the OCD system completely, while the JTAG Programming interface will continue to work the same way as the two other
programming interfaces (HVPP and ISP). To disable JTAG Programming, either set appropriate Lock bits, or use the JTD bit or the JTAGEN fuse.
table shows the relationship between the Fuses and
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