Complexity Is the Enemy
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P018_NELE_APR24_Layout 1 19/04/2012 13:39 Page 18 Technology Watch is sponsored by RS Components isibility is everything in debug, but it is hard to achieve in the world of Complexity is Vembedded systems. Compared to desktop work, where a software monitor can easily show the internal state of a system, the enemy debugging an embedded target can seem more like keyhole surgery. ebugging is, on the face of it, a simple process: looking for For years, the in circuit emulator (ICE) was flaws in software by running the program on the target the staple of mcu users. A specialised connector Dhardware and fixing the errors. made it possible to watch bus activity and Not so long ago, debugging was a matter of plugging in provide shadow regions of memory that would something like an in circuit emulator and watching what happened. help control what the cpu was running. This was In some ways, it was like looking into a room through a fairly large vital for rom based mcus as the emulator keyhole. Today, a combination of functional integration, smaller memory could be changed on the fly, in contrast packages and the advent of multiple processor cores has made the to the fixed memory on the target mcu. process of ‘plugging in’ impossible – the ‘keyhole’ is no longer there State machines in the emulator would and more elegant solutions have had to be developed. extract bus activity data and use it to determine Embedded systems are, essentially, custom designs. Engineers where a cpu was in its execution and what data choose the best pieces from a vast collection of components and it saw. When the state machine saw that a work to increasingly stringent constraints. So it’s no surprise to particular combination of events happened, it find that debugging standards have been fairly thin on the ground. could provide a breakpoint: stepping in and As embedded systems – and microcontrollers in particular – stopping execution and then downloading the became more complex, the electronics world adopted its registers and other state information from the standard fall back position of the proprietary solution; if you had a cpu core. Or it could record the data as Motorola processor, for example, you used background debug watchpoints. mode. Standing alongside these was the other fall back position – As mcus became more complex and began the printf command. to incorporate additional peripherals – such as The automotive industry drove an attempt to bring on chip memory and caches to increase speed standardisation to debug in the 1990s; a move that resulted in and reduce system cost – visibility suffered. ICE Nexus 5001. But while it has support from most leading mcu makers had to buy custom ‘bondout’ versions developers, Nexus 5001 is not in general use. of the target mcu that made available to the The problem is put into focus when you look at ARM and its emulator internal bus address and data lines embedded trace methodologies. Heterogeneous systems make it that would normally be hidden from view. But, hard to apply what is, effectively, a proprietary solution, even as these connections were not through proper though Cortex-M cores are used widely. I/O pads, they did not have the same level of What is certain is that, as embedded systems become more electrostatic discharge protection. Bondout integrated and more complex, effective versions were far more electrically fragile – debug methods will become even more and were expensive to replace when a important. It all comes down to time to misplaced connection resulted in the device market; if you can’t debug your system latching up forever. effectively, you can’t release it. And, today, Although programmers had the option of time to market is the synonym for time to falling back on the standby of printf debugging – money. instrumenting the code to spit out hints about its behaviour on a serial port – this approach Glenn Jarrett, head of electronics was generally prone to timing problems. Code marketing, RS Components that worked fine in debug mode would suddenly break when compiled for production. With operations in 32 countries and 17 warehouses, The delays caused by the printf statements or RS Components stocks 550,000 products from 2500 leading other signals might have masked fatal deadlocks suppliers. It serves 1.6 million customers worldwide, shipping and race conditions. more than 46,000 parcels on the day the orders are received. Embedded programmers got a lucky break rswww.com tel: 08457 201201 in the late 1980s after the IEEE Joint Test Access Group (Jtag) put together a plan to reduce the 18 24 April 2012 www.newelectronics.co.uk P018_NELE_APR24_Layout 1 19/04/2012 13:39 Page 19 Technology Watch Research & Development The quest for visibility As technology becomes more complex, so too does working out what’s going on inside an mcu. By Chris Edwards. cost of test. Chip designers realised that allow signals to pass through. This made it Intel quickly adopted the Jtag port for increased integration was making it tougher for possible to test the continuity between the hardware assisted debug, as well as for board OEMs to test assembled boards at the end of devices on the board without probing the pcb level test – implementing it on the 80486 – production using conventional ‘bed of nails’ traces directly. Designers realised the ability to which pushed other manufacturers to use it. testers – so many functions were hidden away send commands to logic blocks inside a chip Motorola – now Freescale – developed its inside individual pieces of silicon and the chips could not only be extended to test functions Background Mode Debug (BDM) interface, later themselves so tightly spaced that board level inside the device itself, but also to send embracing Jtag as the access port for the BDM probes were unable to reach them individually commands to the logic blocks and to extract logic on its products. However, Jtag to see if they worked. information from them as they ran in a implementations were far from equal. Although The answer lay in providing access to test functioning system. The first target for this many companies put the debug functions on a logic inside each chip through a serial bus that extension of Jtag was the mcu’s central different internal scan chain to the test functions, wove its away around the board and inside processing unit (cpu), which was beginning to some popular devices – such as the IBM each device. Using this bus, a tester could suffer from a similar visibility problem to that PowerPC 600 series – did not, making the job of disable the core logic cells inside each chip and experienced by board level test engineers. accessing the debug support often quite tedious. Since its early days, the x86 had included an Fig 1: The Nexus 5001 standard divides debug support into several classes instruction intended for use by software debuggers. The INT 3 instruction is a single byte instruction – with an opcode of 0xCC – that Static debug forces the processor to run an interrupt. Read/write register and memories Start/stop processor Class 1: basic run control Generally, this would be used by a debugger to Hardware/software breakpoints patch a location in memory to force a breakpoint when the processor hit that location. The use of a single byte made it possible to patch any instruction in the x86 set, some of Watchpoint message which are only 1byte long. The 80386 greatly extended hardware Ownership trace message Class 2: instruction trace watchpoints support for debugging with the inclusion of six debug registers that could be used to set breakpoints without patching memory directly. Program trace messages The registers could be set to any location in memory – allowing breakpoints on data as well as instruction accesses. Another register setting Read/write access made it possible to single step through the Class 3: read/write access data trace code. The processor would halt after every executed instruction. The inclusion of the Jtag Data trace messages port on the 80486 made it possible for external hardware to program these registers without needing to interfere with the running program. Memory substitution Motorola’s BDM brought greater visibility to Class 4: memory and port substitution lower cost mcus and processors, such as its popular 68332, without adding dedicated Port replacement breakpoint registers. Instead, BDM provided an onchip controller for the cpu that could change www.newelectronics.co.uk 24 April 2012 19 P018_NELE_APR24_Layout 1 19/04/2012 13:39 Page 20 Technology Watch is sponsored by RS Components or fetch register and memory contents through Fig 2: An example of debug connectivity in a multicore ARM SoC its serial connection without halting the target, using a mechanism similar to direct memory access (DMA). As a separate piece of hardware, the BDM controller could interrogate the AHB AXI system after a software crash or force the core to AXI to APB bridge bridge into single step mode. Later parts added breakpoint registers to avoid the need to patch memory with trap instructions. AMBA AXI bus Unfortunately, a comparatively slow serial connection is only good for relaying start-stop Cross trigger matrix commands and extracting small chunks of data at a time. One big advantage of the ICE was its ability to provide a real time trace of program execution. Without trace, programmers had to, Cortex ARM dsp again, lace their code with printf statements to core core work out which branches the processor took during execution. Some real time operating interface interface Cross trigger systems, such as VxWorks, added their own Cross trigger level of instrumentation to improve debug visibility – storing a record of recent system calls Debug access in a chunk of memory set aside for the purpose Jtag port – but this again could alter timing related Jtag behaviour.