EECS 473 Advanced Embedded Systems

1. EECS 473Advanced Embedded Systems Bonus lecture Bootloaders, processors vs FPGAs, and PCB wiring issues.

2. Bootloaders • One thing that we haven’t covered in class is how to program a new chip. • Development boards, such as the Arduino, allow you to program the processor. • Most processors have a mechanism for programming the chip

3. Default programming method • Processors need to have some kind of default programming method. Many work hard to make it difficult to enter programming mode on accident • That would be an ugly failure mode.

4. PIC18 example • Drive the MCLR pin with the programming voltage (generally 13V) • Program using clock and data (PGC and PGD) • There is also a lowvoltage programmingmode on some PICS • Can use 5V to program. • Wastes pin, why? • Or clock a special 32-bit pattern via PGC/PGD to enterprogramming mode http://www.best-microcontroller-projects.com/pic-icsp.html

5. JTAG programming • JTAG was originally a port that could be used to do “boundary checking”. • That is, checking to see if the PCB connections between devices were correct. • Could walk each pin and see if it was connected to what it was supposed to be connected to.

6. JTAG continued • Could even use JTAG to test connections between devices. • Darn handy automatic way of looking for wiring errors (bridges, etc.) • Pretty good overview (which seems largely correct and is well written) at http://blog.senr.io/blog/jtag-explained • Though it’s largely about hacking a device via jtag.

7. A lot of devices allow you to program them via the JTAG port • Not much to add other than each device tends to be a bit different. • TI has a nice document here: • http://www.ti.com/lit/ug/slau320x/slau320x.pdf • FPGAs also often are programmed via the JTAG port. • http://microblog.routed.net/wp-content/uploads/2006/11/xilinx-jtag.pdf • Exact scheme and information tends to vary wildly by manufacturer and often by processor/FPGA family. • Basically opens up a programming/debugging window to the hardware.

8. How else can you program a device? • Well, you could program the device once with a program that waits for data input on reset. • If it sees the data, it programs the chip with said data. • If not, it just runs whatever program it has. • Called a bootloader.

9. Bootloader overview • Bootloaders are fairly simple • Generally wait for input on some serial port • If no input, timeout and jump to application • Otherwise program chip with incoming data. http://www.microchip.com/stellent/groups/sitecomm_sg/documents/devicedoc/en558478.pdf

10. Some bootloaders are very complex • Intel has a whole set of bootloader development kits. • Lots of components to turn on. • Lots of configurations to walk through • In part due to backwards compatibility of x86. • The “minimal bootloader” documentation is 25 pages long and discusses: • Reset vector handling, preparation for memory initialization, memory initialization, post memory initialization, interrupt enabling, processor intterup modes, timers, caching, I/O devices, and more. https://www.cs.cmu.edu/~410/doc/minimal_boot.pdf http://www.intel.com/content/www/us/en/intelligent-systems/intel-boot-loader-development-kit/intel-bldk-initialization-firmware-development-solutions-toolkit.html

11. Moral of the story • When designing a PCB, you need to be sure you can program your chip. • In some cases it makes sense to program the chip with a bootloader before you solder it. • In some cases you need special programmers. • SPL has PIC and Atmel programmers. • Often require special voltages or other magic.

12. An overview of embedded processors Microcontrollers, DSPs, System-on-a-chip (SoP), ASICs and more

13. Processing unit overview Where to start? • When designing an embedded system, one key question is: • “What processor should I use?” • Obviously the answer depends on a massive number of factors. • CPU computational power needed • And even the type of computational power needed • Interfaces needed • Cost • Power • Expected design time • Debug capabilities • Current familiarity with tools • Etc.

14. Processing unit overview Highest level: ASIC, off-the-shelf processor, FPGA • These three categories are pretty straightforward and mostly non-overlapping. • An ASIC is an Application Specific Integrated Circuit. • You are “spinning” a chip designed specifically for your application. • Manufacturing costs are a bit difficult to figure out • MOSIS would charge $10,000 for an extremely small chip (40 chips, 1 square mm each) in a very large (700nm) process. • I’ve seen groups pay prices of $50K for a couple of larger ARM-based chips. • Design costs are yet more. • I know one company spending “high single-digit millions” for the design and manufacture of a few thousand chips • Of which they will probably only use a few hundred. • So: • Pros? • Cons?

15. ASIC ASIC • Not a lot to say here, each case is, by definition, different. • A lot of embedded systems work is with ASICs these days. • The (vast?) majority of cell phones use in-house designed processors, 95%+ of which are ARM based. • The iPad uses an ASIC. • What’s interesting is how design costs generally dominate over the one-off mask costs • Not that mask costs aren’t a major factor. • You really want to avoid respins… • If you have a large market, need decent performance and are highly power constrained • ASIC is clearly the way to go.

16. Processing unit overview Highest level: ASIC, off-the-shelf processor, FPGA • An off-the-shelf processor is one that has been manufactured by someone else. • This doesn’t mean that the processor isn’t highly specialized. • There are chips designed just for printers for example • the HP Unity is MIPs based, ½ GHz, ½ Gbyte standard • Pros? • Cons?

17. Processing unit overview Highest level: ASIC, off-the-shelf processor, FPGA • You all know what an FPGA is. • But when would you choose to use one in an embedded system? • Even if it isn’t in your embedded system, why else might an FPGA be helpful to embedded system designers? • Pros? • Cons?

18. FPGA FPGA • There can be quite a bit of feature difference between the different FPGA choices. • You can find FPGAs for as little as $10 and as much as $3000. • Some use non-volatile memory, others need external help on power-up.

19. Processing unit overview But at this high level, things are fuzzy • Sometimes an FPGA and off-the-shelf processor are integrated. • Actel in 373 • Cypress Semiconductor • Their fabric is generally a bit less flexible than an FPGA, but easier to work with. • And many ASICs are based on standard “off-the-shelf” architectures • ARM being the most common these days • MIPS was similar at one point.

20. Off-the-shelf What device options are there? • One fun part about this is that there are a number of terms used to describe the options, but they aren’t very well defined. • For example, what’s the difference between a microprocessor, a microcontroller and a System-on-a-Chip (SoC)? • It’s really not clear • One axis is that a SoC is expected to have a richer set of peripheral interface capability More peripherals and interface capability SoC Micocontroller Processor

21. Off-the-shelf However the terms also carry other connotations • For example, in the “CPU-capability” axis microcontrollers are generally thought of as being very limited, generally in the 1-30 MIPs range. • And correspondingly, the microcontroller is expected to have lower power needs. • So keep in mind that terms aren’t always well-defined! Processor SoC Micocontroller CPU-capability

22. Off-the-shelf So… • While those terms (SoC, Microcontroller, Processor) are commonly used • They aren’t all that well-defined. • None-the-less, we are stuck with them • So let’s explore a bit…

23. Off-the-shelf Microcontroller • The AVR processor is an example of a microcontroller • Flash, SRAM, EEPROM • 6-30 GPIOs • 4-20MHz • Basic serial I/O capability

24. Off-the-shelf Example SoC: Cypress PSoC3: CY8C38

25. Off-the-shelf Example SoC: Cypress • It looks similar to the AVR processor • A bit faster for sure, but when you are advertising multiply and divide… • Power: • How long would a 2000mAh battery be able to keep this thing running? • But it’s peripherals are much more significant. • 68 GPIOs, • Highly configurable on the fly. Can have: • Built-in hardware blocks for FIR/IIR filters @67MHz • Has 4 built-in op-amps (less parts on board) • Can directly interface to a capacitive touch screen

26. Off-the-shelf Just the analog…

27. Off-the-shelf Processor • When people discuss processors they often mean something like a desktop CPU. • Power in the 20-200W range for example, but speeds in the 2-3 GHz range. • However there are things like the Atom core (which, Intel markets as a SoC…)

28. Off-the-shelf Intel AtomWhat looks different?

29. Off-the-shelf What type of specialized off-the-shelf processors are there? • Generally speaking, you are looking at either • CPU instructions and features focused on improving performance on a specific type of application • DSP and encryption both have very specific needs. • Specialized I/O. • So CAN bus (standard automotive shared serial bus) • Capacitive touch screen interfaces

30. A bit on power • There are a lot of things we could talk about with respect to power issues. • Easiest to screw up are probably ground issues • We think of “ground being ground” but wires on our board that claim to be ground aren’t exactly at the battery/Power supply ground. • Further, EMI/EMC issues can exist.

31. Care with grounding • Return paths. • Ground isn’t ground. • This is an important rule. • Say that “A” is a processor 3.3V@100mA and “B” is a motor 12V@10A. • Say that the wire between A and the power supply has a resistance of .05Ω(1 oz/ft2, 20 mil, 2 inch) • What is the voltage at A’s ground? 3.3V 12V 0.1A 10A 0.05Ω

32. Ground loops • Basic theme: • Use a ground plane. • Be sure the plane runs under any high-speed signals. • If you can’t use a ground plane, be sure your high speed signals have a return (ground) wire right underneath them. • Nice 3 page paper at http://www.ultracad.com/articles/loop.pdf