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CPU Angst

I’m starting to fear that Tiny CPU is a short-sighted design. It occupies a strange spot between ultra-simple soft-CPUs like MCPU, and much more capable soft-CPUs like PicoBlaze. The hardware is nearly as complex as PicoBlaze, but the functionality is not so much better than MCPU, making it a poor trade-off. The 10-bit address space and lack of true pointer support (no generic indirect addressing mode) are especially limiting.

The project is far enough along that I’ll finish it anyway, so it will be a good learning experience if nothing else. Next time if I do a “Small CPU” project, substituting even a very small FPGA for the two CPLDs I’m using here should allow for dramatically improved functionality.

Supply Current

I took some power measurements for my test board, at various clock speeds and with different peripherals connected.

  1.22 MHz 4 MHz 8 MHz 20 MHz
with keyboard and 20×2 LCD 142 mA 147 mA 145 mA 148 mA
with LCD only 141 mA 143 mA 144 mA 147 mA
no peripherals 89 mA 100 mA 101 mA 105 mA

Conclusions: Clock speed and the presence of a keyboard have little effect on the current draw. A 20×2 line text LCD adds about 50 mA. I also tried a 20×4 LCD, which draws about 65 mA.

The LCD current is higher than I expected. The datasheet for the 20×4 LCD says the supply current should be just 4 mA, but I assume that’s without the backlight. The datasheet doesn’t say much about the configuration of the backlight LEDs or their current needs, and whether they’re in series, parallel, or both, except to note that VLED is 4.2V. That probably means there are two LEDs in series, or possibly several series pairs, connected in parallel.

As I type this, I realize I’ve made a mistake. If the backlight drops 4.2V, then 0.8V should be dropped across my current limiting resistor. If I want 15 mA (pretty typical for an LED), then the resistor should be 0.8V / 15 mA = 53 Ohms. But I’m actually using a 15 Ohm resistor, and I don’t remember how I came up with that value. With that resistor I should see 0.8V / 15 Ohm = 53 mA going to the backlight, plus 4 mA for the controller chip, so 57 mA. That’s not far from what I measured, so at least that makes sense.


I also did some experiments generating tones with a simple speaker. Directly driving the speaker with a digital output pin works, but draws about 15 mA with a 50% duty cycle square wave. Using the digital I/O to switch a transistor on and off instead delivers 50 mA to the speaker, making it noticeably louder while also reducing the current demands on the CPLD. It does require adding a handful more passive components to the board, though: in addition to the transistor itself, it needs a resistor to limit the base current, and a flyback diode in parallel with the speaker to discharge reverse voltage created by switching.

Serial Port

Next up: working on a soft-UART serial port, for communication with a PC. I’m not sure how that will get used ultimately– maybe a bootloader? I’m sure it will prove useful, though.

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