Differences
This shows you the differences between two versions of the page.
| Both sides previous revision Previous revision | Next revision Both sides next revision | ||
|
en:reverse-engineering:jtag:start [2013/02/16 09:30] alex [Links] |
en:reverse-engineering:jtag:start [2013/02/18 20:46] alex [Output probing] |
||
|---|---|---|---|
| Line 123: | Line 123: | ||
| ==== Output probing ==== | ==== Output probing ==== | ||
| - | Output probing walks a toggling output pin around the board while at the same time printing the name of the pin to STDOUT. This is not terribly useful in and of itself, but in addition to an oscilloscope with serial decode capability, output pins can be traced. The simplest way to set this up is to pipe the output of ftjref running an output probe to a serial port, and then connecting one of the oscilloscope probes to the serial port and enabling serial decode. Put the oscilloscope in normal trigger mode to trigger on any edge on a free probe, and then use this probe to browse the board. When the probe picks up a JTAG triggered edge, the serial decode displayed alongside will correspond to the connected pin. Sometimes multiple pins will trigger the same edge; it can take some work to determine the precise cause. Output probing can sometimes determine what pins driven from external sources are connected to, but this does not always work. If the non-JTAG device's driver is weak enough, the JTAG controlled driver may be able to produce enough of a change in the line to detect on an oscilloscope. | + | Output probing walks a toggling output pin around the board while at the same time printing the name of the pin to STDOUT. This is not terribly useful in and of itself, but in addition to an oscilloscope with serial decode capability, output pins can be traced. The simplest way to set this up is to pipe the output of ftjrev running an output probe to a serial port, and then connecting one of the oscilloscope probes to the serial port and enabling serial decode. Put the oscilloscope in normal trigger mode to trigger on any edge on a free probe, and then use this probe to browse the board. When the probe picks up a JTAG triggered edge, the serial decode displayed alongside will correspond to the connected pin. Sometimes multiple pins will trigger the same edge; it can take some work to determine the precise cause. Output probing can sometimes determine what pins driven from external sources are connected to, but this does not always work. If the non-JTAG device's driver is weak enough, the JTAG controlled driver may be able to produce enough of a change in the line to detect on an oscilloscope. |
| Example: | Example: | ||