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I'm working on a little side project with the LPC810 (Cortex M0+ 32 bit ARM MCU, 8 pin DIP package, 4kB flash, 1kB SRAM). I need a UI to convey to the user a temperature setting. I only have about 2 pins spare to implement the UI.... so using LEDs is going to a challenge.

I had an idea of using just one pin to drive a speaker or peizzo buzzer and convey the temperature in speech. Just the digits 0 - 9 in english is all I need. Storing 0-9 in PCM is out of the question with so little flash. So I was wondering is anyone aware of open source code that would allow for very high compression of a small dictionary of words what would be light enough on CPU/RAM to work with a Cortex M0 class MCU? (I have a strong hunch this is plain out of reach of low end Cortex M0 MCUs without external memory).

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    \$\begingroup\$ Not a direct answer to your question, but with 2 pins, you can drive many LEDs. Ether use shift registers or I2C to GPIO chips. \$\endgroup\$ – DoxyLover Jul 28 '13 at 17:25
  • \$\begingroup\$ What I'm doing here is more a challenge to see what can be done with little or no extra hardware... I'm not necessarily going for the most pragmatic solution :-) You're absolutely right: there are loads of serial devices I could attach for UI. \$\endgroup\$ – jdesbonnet Jul 29 '13 at 0:36
  • \$\begingroup\$ FYI, I just posted a variant of your question as a challenge on codegolf.SE. Perhaps some of the (hopefully forthcoming) solutions there may be of interest to you as well. \$\endgroup\$ – Ilmari Karonen Aug 2 '13 at 13:55
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While this may not be a complete, independent and ready-to-use answer, but I think you can get some neat ideas, implement (or port) it on Cortex-M0. Here I assume that from a computation power and resource standpoint, a Cortex-M0 has more to offer, than the popular Atmel 8-bit AVR (ATmega328P) running on an Arduino.

Here are 2 projects, that manage to use the PWM pin of Arduino and an RC-filter circuit to play out synthesized speech. Of course, we are not looking at hi-fidelity audio, but something that is recognizable. Also do note, that apart from the need for a PWM capable pin, your micro-controller might be very busy during the synthesis, so much so that, it might spend most of it's cycles doing it. Software PWM would put further strain.

Now for the 2 projects:

PS> Personally, I've not implemented them, but looked at them for a project.

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I need a UI to convey to the user a temperature setting. I only have about 2 pins spare to implement the UI.... so using LEDs is going to a challenge.

Use the two pins as I²C Bus, and connect a GPIO expansion chip like MAX6956, which has 20 output pins in a 28-pin DIP package. That is enough for a few 7-segment displays, and you can connect more than one expansion chip to the bus.

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    \$\begingroup\$ Thanks for the suggestion... this is a challenge make a sous vide cooker from nothing other than a hacked power timer, DS18B20, LPC810 (8 pin DIP) + passives. So what you're suggesting makes perfect sense normally, I'm actually deliberately going for weird and minimal. \$\endgroup\$ – jdesbonnet Jul 29 '13 at 0:44
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It may be possible to write code that would fit in 4K that would produce vaguely-recognizable speech for the digits 0-9, but it would probably be easier to define beep patterns for them. A small amount of training might be required to recognize the beep patterns (among other things, if you're using a positional notation, knowing how a zero would be conveyed) but if all temperatures are positive it may not be too hard to define beep sequences for values 90-10 and 9-1.

+90: XXXXX--XX--XX---
+80: XXXXX---X-X-X---
+70: XXXXX-X-X---
+60: XXXXX-X-
+50: XXXXX---
+40: XX--XX--
+30: X-X-X---
+20: X-X-
+10: X---

Each "X" represents 50ms on and 50ms off, while each "-" represents 100ms off. For values 9-1, use the same cadences as 90-10 but a different pitch. The cadence patterns above could easily be stored in an array of 16-bit integers. The sequences above are chosen not just to allow counting, but also to be recognizable as patterns. Counting straight non-rhythmic pulses up to nine can be a bit difficult, but arranging the things to be counted into rhytmic grouping can help a lot.

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  • \$\begingroup\$ Yes, this is a good idea. My original plan was to blink a LED using a similar code (the project is a sous vide cooker controller... and the temperature range is actually very narrow... about 55C - 65C). Thinking that maybe I could double up functions: both LED and buzzer on the same GPIO line. Instead of solid on for the LED, I send a 50% duty cycle audio tone... indistinguishable to the eye. \$\endgroup\$ – jdesbonnet Jul 29 '13 at 0:48
  • \$\begingroup\$ I think hearing tones could be easier than observing blinks. I would also think, though, that you might want to be able to have the LED on while the device was silent. Depending upon VDD and power-draw requirements, you may be able to arrange things so driving a pin one polarity hits the LED, and driving it the other way hits the piezo (float the pin to activate neither the light nor the piezo). \$\endgroup\$ – supercat Jul 29 '13 at 16:44
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If board space is not a limitation, A port expansion + few counters (74HC161 or similar), external sample ROM and a DAC can implement this without requiring much code space.

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