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5 stories in 2015 you should know

Posted: 28 Dec 2015     Print Version  Bookmark and Share

Keywords:connected devices  RISC  CISC  timers  FETS 

A look at timers

When I decided to write about timers, it seemed like it would be a simple exercise. Timers are one of those components that straddle the digital/analogue divide and I just wanted figure out a definition that would place my discussion in the corner of the latter. The different types left me completely confused as to how to categorise and group them. I was hoping to refer to devices that I might use in a simple circuit and as a dividing line I decided that the device should self-oscillate (intentionally, of course). As I result I have left out oscillators including crystal and pre-programmed oscillators. I have also ignored devices that would connect to a microcontroller and watchdog timers. Most devices seem to have a square wave output.


Figure 1: Simplified schematic of the 555 timer (Texas Instruments).

Perhaps the most successful chip of all time is the 555 timer (block diagram above). In this day and age where the number of manufacturers of product families like the CMOS 4000 series has decreased to only 2 or 3, there are still many manufacturers of this bubble-gum part so there is obviously still profit in it. If you factor in the derivatives the 555 market must be absolutely enormous. Not only is there the 556 (dual 555) and the 7555 and 7556 (CMOS versions), but there are devices that offer lower leakage currents like the ALD1502 from Advanced Linear Devices or the ZCST1555 from Diodes Inc. which will run at a very low supply voltage. Throw in the Micrel MIC1555/1557 series which reduces the number of pins and you get start to get the feeling that the 555 will be the device use to turn out the lights when civilisation ends.

I don't use Voltage Controlled Oscillators (VCO) often and most that I have come across are a bit long in the tooth. There is one embedded in the CD4046 Phase Lock Loop that you could use, but there is also the Exar XR2209 which has both a square and triangular output. There are also some TTL parts like the 74LS624, 74LS628 and 74S124 and (shudder) some ECL parts as well. The only newer part is from Linear Technologies in the TimingBlox family of parts. The LTC6990 is a VCO where the frequency decreases as the voltage increases.

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