Introduction
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As if we needed another protocol
These days it seems like everyone has their own way of doing things. When it comes to programming a microcontroller, that statement still holds true. If you are like me, then when you find something that works you keep using it until you can’t because that makes your life easier. I have gotten used to using a JTAG connector and while I am not an expert, it feels comfortable and I like that feeling.
In the past few months, I was working with the engineers at COG9 (follow the link to get to their website) in SLC and they were programming an MSP430 for a project we were working on using Spi-Bi-Wire. When hardware and firmware folks work together, it is very useful if both parties have hardware and both can independently program the Flash. While this was not a very difficult task, it was challenging and I thought it might be useful to go over how I did it for the first time so it will be easier if you decide to jump in as well.
What is Spy-Bi-Wire?
As the name implies it is a two-wire interface (if you don’t count power and ground, which you will need whether you count them or not). It is a serialized JTAG protocol developed by Texas Instruments for their family of MSP430 microcontrollers. You will see two SBW pins on these microcontrollers, SBWTCK (Spi-Bi-Wire Test Clock) and SBWTDIO (Spi-Bi-Wire Test Data Input Output).
The protocol uses time-division multiplexing (right?!) and allocates three timeslots, TMS_SLOT, TDI_SLOT and TDO_SLOT. That is probably more than we need to know unless you plan to make your own programmer. I used the MSP-FETFlash Emulator Tool (the black box). It has a 14-pin output connector. You are, of course, only going to use 4 of those.
Let’s get dirty
The first pin is the easiest, ground. That is pin 9 on the ribbon connector. Next you need to connect power. Here you have a choice. If your target board has its own power you will connect it to pin 4 of the ribbon connector (VCC_TARGET). If on the other hand you want to have the programmer to provide power you will connect the VCC of your target to pin 2 of the ribbon connector.
Next comes the fun stuff. Connect SBWTDIO to the TDO pin, pin 1 on the ribbon connector. And finally, connect SBWTCK to the TCK line, pin 7 of the ribbon connector. As you can see from the picture in figure 1, I hacked one of the adapter boards I talked about last week, but you could easily do this with a perf board of some kind.
Figure 1 SWD programming setup
I am sure there are many ways of programming the MCU, but I was provided with a program.hex file. I downloaded the MSP430Flasher, which is an executable so be prepared to run it out of a command window. After opening a command window, and pointing it at the MSPFlasher directory where I had also placed my copy of program.hex, I issued the following command: msp430flasher.exe -w program.hex. I got an error. It did not work!
Mayday-Mayday
I realized that when I use the regular JTAG programmer with an IDE, I always start by erasing the chip (because if you don’t it usually does not work). So, I did that and got the same error. As I poured over datasheets and web pages I found a note that said to make sure that the distance from the target to the programmer did not exceed 20 cm. If you take a look at figure 1 it will be obvious I had made a boo-boo. The ribbon cable alone is that long. As I looked at the available instructions in the command window I spotted my salvation, the ability to control the transfer rate (default is medium speed). I issued the following commands:
Msp430flasher.exe -j slow -e erase_all
Msp430flasher.exe -j slow -w program.hex
I had to cycle power (as indicted in the datasheet) but that was it. I had just used a 4-pin connector to program an MCU. Pretty neat. And I didn’t have to go and beg the firmware guy to program my hardware (they hate that you know).
Now what?
The next step is, of course, to make a better interface for the next time. I will make a small board that has a 14-Pin keyed connector on one side and a 4-pin connector on the other. I will make a shorter 14-pin ribbon connector and I will make sure that the 4-pin cable or connector, as well as the 4-pin connector on the PCB, are clearly marked so I know the orientation to plug them in.
I hope you found this useful. If you give it a try, please let me know about your experience.
Final thoughts
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