Microcontroller Showdown, Conclusion

The last chip in the comparison is the Z16F2810. When I started the tests I got a Loop overhead time of only 0.64, which made me think the chip might be very fast. When I moved on to the multiplication and division tests, though, some of them completed in only 0.89 seconds. After looking at the assembly generated during compilation I noticed that the arguments of division were only being copied to the register that holds the answer. It seems that even though all the variables were declared as volatile, the compiler decided to optimize the division out anyway. This made me think the compiler was simply ignoring the volatile keyword but the assembly generated when accessing volatiles is slightly different. Loops involving them take about 25% longer to run, so it does have an effect. The only way I could prevent it from optimizing out multiplies and divides while keeping optimizations was to declare the involved variables globally. The User Manual for the chip helped a lot when looking at this problem and I think it is really well done.

For arithmetic operations, the chip did very well. It was always faster than the MSP430 and almost as fast as the LPC1114 for most things. One place it did very well was division where it outperformed all the other chips by a large margin because it has a hardware divider. General GPIO is also pretty fast, although it does not have any special mechanism for speeding it up like the masks on the LPC1114 or the constant generator on the MSP430. BCD Add was about the same as the MSP430. BCD Multiply was faster than on the MSP430 but still much slower than on the LPC1114.

After running all the chips, here are my conclusions:

• MSP430 Easy to use and reasonably fast. Good choice for small jobs and low-power projects. Held back by low clock speed (16MHz), small memory, and lack of hardware multiplier.
• LPC1114 The fastest chip by far for everything but division and 8 bit math. Lots of RAM and Flash. Straightforward to use, although there is no real community. Very fast GPIO with masks. Top choice for calculations.
• DS89C450 Fast for 8 bit math, although slow at shifting. Very fast GPIO since each GPIO pin is mapped to its own byte in memory. Extremely slow at BCD calculations. The only real advantage is the external memory bus.
• AT89LP6440 Similar to the DS89C450 but lower clock speed and performance. 
• Z16F2810 Good performance but inferior to the LPC1114 in everything but dividing. Debugger is very useful but the IDE itself is sometimes clunky. Enough RAM and very big Flash (128KB). PLCC package is inconvenient.

In the future I plan to do a few more projects that will need the extra horsepower for the LPC1114. For other smaller jobs I will stick with the MSP430.

Results are after the break.

Microcontroller Showdown, Part 4

The next chip to test is the AT89LP6440. This is a single-cycle 8052 compatible like the DS89C450. It can run from an internal oscillator at 8MHz or an external crystal up to 20MHz like I am using. According to the datasheet, total capacitance for the crystal should not exceed 20pF and the chip itself adds about 10pF of that. The smallest capacitors I can find are 22pF so I ran it without capacitors and it seems to work fine on a breadboard.

As you might imagine, the AT89LP6440 performs similarly to the DS89C450 at 20MHz. The times are not exactly the same, though, because both chips perform some instructions in less cycles than original 8051s. This is apparent, for example, in the multiply times where the AT89LP6440 can do 32 bit multiplication about 30% faster because it does multiplies in 2 cycles instead of the 4 cycles needed by the DS89C450 and standard 8051s. The AT89LP6440 also shifted about 5% faster in the tests, although it was a little slower at BCD calculations. Like the DS89C450, I don't think I will end up using this in a calculator but it may come in handy because it can run at 3 volts and has an external memory interface.

Results are after the break.

MSP430-Based AT89LP6440 Programmer

Using an FTDI cable to program the AT89LP6440 like I mentioned in my last post turned out to be more difficult than I expected. Configuring the FT232 chip is a real pain because the reset command in the API does not seem to actually reset any of the chip settings to default. After trying some more tweaking I noticed that the uploading itself inexplicably hangs sometimes on longer files. I was not able to finish the next step of the microcontroller comparison. According to another page, FTDI states that "using bitbanging for SPI or I2C is not recommended with the FT232RL part."

The next step was to use an MSP430 to take data in over UART and relay it over SPI to the AT89LP6440. The UART interface of the DS89C450 works really well so I copied this. The MSP430 gives a prompt over UART that can be used to erase the chip, load or verify firmware, and set flags. The challenge of the project was coordinating interrupts and ring buffers for the hardware UART and SPI peripherals so that no data is missed. At 9600 baud it worked reliably but at higher speeds there were checksum errors because sometimes the previous UART byte was not read before the next one came in. This seems to happen at the end of a line of hex when the MSP430 has to send a lot of data out over SPI to prepare the chip for writing. The solution was to send an XOFF character back to the terminal, which stops it from sending any more data bytes until an XON character is sent. Like this I can program at 57.6k and after a few dozen tries I have not had any errors writing to the chip or verifying the firmware. The MSP430 is also connected to the reset line of the AT89LP6440 so that I can put the chip in run mode without having to press any buttons on the breadboard.

The source for the programmer can be found here: AT89 Programmer.

AT89LP6440 Flashing

For the Microcontroller Comparison I have been trying to get the AT89LP6440 to work. When I sampled the chip I thought it could be programmed over serial with an FTDI cable like the DS89C450, but it turns out that it needs a parallel port. There are adapters and programmers that will simulate a parallel port over USB but I don't want to buy one for a chip that I might not use much in the future.

As a workaround I used the FTDI cable to bitbang the SPI interface. This was tricky because I had to write a whole new loading program instead of relying on the program Atmel supplies. Thankfully, they provide very good documentation of the ISP specification. Bits are sampled on MISO on the falling edge of the clock and MOSI is sampled on the rising edge. My first version of the program worked well but could only upload about 100 bytes of firmware per second. Part of this is due to reading MISO every clock cycle, which is not always necessary. By splitting reading and writing into different functions and sending pin states in groups instead of individually, flashing is much faster. There is a bug, though, that randomly gives a wrong byte when I try to read from the chip to verify the firmware. The wrong byte is always at a different address and sometimes the verification completes, so I know the AT89LP6440 has been programmed properly. I wasted about a week trying to tweak the program and saw that the D2XX drivers supplied by FTDI are unreliable and pretty buggy (at least when used under Windows 7 for bitbanging). This particular problem could be with the chip but several of the other D2XX functions I tried didn't work very well either. The same problem happens with both synchronous and asynchronous modes. There is an open source library called libFTDI but I didn't get very far with it because installing the driver for it would mean losing the virtual COM port you get with the FTDI driver. The next step would be to program the chip using another microcontroller connected to the PC over UART but I don't want to waste any more time on this chip since I am unlikely to use it for anything after the microcontroller comparison.

You can download the program here: AT89Loader. Flashing seems to work with no problems but verification occasionally fails.

Microcontroller Showdown, Part 1

Recently I bought some LPC1114 microcontrollers from NXP. They come in DIP packages and feature an ARM Core M0. Although comparing the clock speeds of different architectures is not the best indicator of speed, I would still expect this chip running at 50MHz to be faster than the MSP430s running at 16MHz that I have been using until now. Later I found out that the flash of the LPC1114 only runs at 20MHz, which will bottleneck any programs running from there. Running from RAM would be much faster but the chip only has 4KB of SRAM. On the other hand, it does have a three-stage pipeline. Value-line MSP430s only have instruction-prefetch. All of this makes it a bit difficult to know how much faster one chip would be than another, so I have decided to run a few benchmarks.

Another architecture I have been interested in lately is the 8051. This is much older than the other two architectures I want to check but it remains popular with some people. Instructions take 12, 24, or 48 cycles to execute, which is less efficient compared to an MSP430 which needs only 3-6. Some modern 8051s feature a full-pipeline and clock speeds over 30MHz. These are interesting to me because I think they may be able to outperform the MSP430 in some ways. I was able to sample some AT89LP6440s from Atmel and a DS89C450 from Maxim Integrated which both come in DIP packages. The AT89LP6440 can fetch one byte per clock cycle from memory, meaning that it should in practice run slower than 1 MIPS per MHz but still much faster than other 8051s. It does have a hardware multiplier which surprised me to find out. The datasheet for the DS89C450 lists it as one clock-per-machine cycle, so it should run faster MHz per MHz than the AT89LP6440. It can also run at 33MHz compared to the AT89LP6440 at 20MHz.

The last microcontroller I will test is the Z16F from Zilog. This is a 16-bit CISC design descended from the 8-bit Z80. According to the Zilog website, it outperforms many RISC designs in the same class. It can do 32-bit ALU operations, has a hardware multiplier, and does some pipelining. I know the least about this chip but the design is interesting and I may use it in a future project if it performs well.

Here are some specifications of the microcontrollers that will be involved:

Microcontroller	Clock Speed	RAM	Flash	Core
MSP430G2553	16MHz	0.5KB	16KB	MSP430
LPC1114FN28	50MHz	4KB	32KB	ARM Core M0
AT89LP6440	20MHz	4KB	64KB	8051
DS89C450	33MHz	1KB	64KB	8051
Z16F2810	20MHz	4KB	128KB	ZNEO

There are many factors that need to be taken into account when choosing a microcontroller. For this test I have decided to test only calculation speed and to ignore other factors such as the peripherals. Consequently, the results will not be relevant to some people. This also will not tell me which microcontroller is the best overall. These particular chips are dissimilar in many ways and this is not a comparison of microcontrollers from the same class. They are only being compared because I am curious about them.

One good reason to do this test is so that I have an excuse to set up the tool chains and programmers for all of these chips. This will take some time so I will release the results in a few parts. It will give me a break from the RPN Scientific Calculator I am making.