To co-inside with the results of the 555 contest ( 20th April 2011 - 9pm EST ) I have decided to release what I said I would do. That was to look at what is better, The 555 Timer IC or a Microcontroller? If you do not know about the contest then you should head over to the site and see the great work that's been done and also see what prizes are up on offer!
element14 are proud sponsors of the contest and are giving away a full copy of the Eagle PCB design tool along side lots of others including my own firm ebm-papst UK Ltd who are giving away a fluke meter.
Back at the beginning of the year I was online when Jeri Ellsworth was hatching the idea of the 555 contest. One of the things that was commented on was that a Micro could do everything a 555 could, and better. So over the last few months while everyone has been building there contest entries I’ve been looking into seeing if a Micro can really out do a 555.
I started off by looking at the LM555 and the NE555 data sheets and picking out the key features. These were as follows (other ICs may differ):
Supply voltage of 4.5 to 16 volts, 3mA at 5V, 0-70’C working range, 8 pin and about 100ns rise / fall time on output capable of 200mA. These devices also cost between 42p and 62p (UK 1off prices).
So then it came to select a microcontroller - I’m a big fan of Microchip so had a look at the smallest chips they have. I started with the PIC10 range but after looking at pin outs, the device even when in a 8 pin package only used 6 pins, so could not match all the pin functions of the 555.
So my final choice was the PIC12F615 that has the following Spec:
Supply 2.0 to 5.5 volts, 1.1mA at 5 volts, -40 to 85’C working range, 8 pin better than 100ns rise / fall response time with 100mA output pin. cost wise I found the device for around 71p.
So how did it compare - well first off the PIC12F615 is not pin compatible, so you can’t go swapping out a 555 for a PIC. Its also very obvious that the working voltage ranges are very different. The 555 is a good fit for analogue circuits as well at logic 5 volts, but the PIC is a pure logic device - obviously! While on the supply side of things - we can see the PIC will draw less current than a standard 555, but I have found 555s that will work with less current since my initial selection.
Temperature is always an interesting question with a 555 as they are know to drift at extremes, however the PIC is much more stable and also has a better temperature range. You could spec in the SE555 which will match the PIC on temperatures, but then PIC could always play the +125’C card and still out do it.
Functional wise the PIC worked really well and I got almost identical results from the standard circuits show in the data sheets. This was a Monostable with a 1ms output and a astable of 1kHz and 75% duty. In fact at 5volts there was nothing to select between them. However the PIC took programming time and is consumed with just doing this role. Also I felt the ADC inputs would not as sensitive as the comparators in the 555’s as I was only seeing +/- 19mV steps.
I then for may last tasked asked myself, could I use my PIC12F615 in the projects I have seen entered in the 555 contest. To be honest I’d have to say no. Why? well the voltage the PIC can work at just did not match that of some of the entries, its not sensitive in my view and its more expensive and difficult to use as you have to generate code.
Therefore I can say that Jeri was 100% right in saying that the 555 can not be replaced by a microcontroller. Yes you could in some cases and it may even be better in some cases but on the whole there is no real reason to use anything other than a 555 Timer IC.
I've not seen the results of the contest at the time of writting this blog post - but want to wish everyone the best of luck and to say that the entries have all been really good.! They have been great to look at and have given me lots of ideas for project os my own. Well done!
