Page 156 - The Art of Designing Embedded Systems

Troubleshooting Tools 143

I data bus

clock
serial-in
serial-out
FIGURE 7-2 A BDM/JTAG debugger adds logic on the CPU itself.

is inherently not coupled to raw processor speed. Connection problems go
away, since you just run a few CPU pins to a special debug connector.
Implementations vary, but a processor with BDM dedicates a few
pins to a serial debugging channel (though sometimes other functions
might be multiplexed onto them). Customers demand high-speed screen
updates, so this is a synchronous communications scheme that includes a
clock pin, supporting serial speeds beyond 1 Mbps.
Development tool vendors sell you a connection to this channel,
ranging from a high-end very fast link to something no more complicated
than a two-IC interface to a PC’s comm port . . . and, of course, a source-
level debugger. The software interfaces to your code and formats your re-
quests to single-step or display data to meta-commands transmitted to the
CPU chip (on the BDM link).
The original BDM implementation shared microcode with the proces-
sor’s main execution stream. Commands processed by the debug link thus
stopped normal program execution. Although this was tolerable for simple
applications, users of real-time operating systems, in particular, wished to
examine and alter system state without bringing the entire program to its
knees. BDM+, on the ColdFire CPUs, uses a totally independent set of
hardware to allow concurrent program execution and debugging.
MIPS, Intel, TI, and others provide serial debugging via various ex-
tensions of the JTAG (Joint Test Access Group) standard (IEEE 1149.1).
JTAG, too, is a synchronous serial interface, one originally defined to pro-
mote testability of complex boards. Though the implementation details
differ from those for BDM, in all significant user respects it offers the
same sort of functionality and level of complexity.
BDM and JTAG hardware on board the processor can’t waste tran-
sistors, as ultimately increasing the chip’s complexity drives the cost of the