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A look at timers

Posted: 14 Oct 2015     Print Version  Bookmark and Share

Keywords:timers  circuit  self-oscillate  oscillators  watchdog 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/analog 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: 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 civilization 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.

A logical extension of the VCO (to me anyway) is the voltage controlled Pulse Width Modulator (PWM). I know with the small micros that are available today, a PWM controller is relatively simple to implement replacing the component-intensive method of generating it with a single chip. However if you don't want to get into coding and debugging and the expense of programming each and every micro the LTC6992 TimerBlox Voltage Controlled PWM will fit the bill.

Sometimes you want an oscillator without having to set up a 555, or you want a higher frequency or better performance. There are several devices that will help you. Maxim has their EconOscillator range including the resistor programmed DS1090 . There is Touchstone's (now Silicon Labs) selection which is also resistor programmable and, depending on the device, will go from 4Hz to 300KHz or from 1.7mS to 33 hours! Linear Tech also has a contribution to this space- the LTC6991 which uses resistor programming to go from 2mS to 9.5 hours.

And then there are the oscillators with counters to extend the timing function. Let's start with the historical units- there is the CD4060 with a 14-stage binary counter, the CD4541 with a 16-stage counter, the CD4536 with a 24-stage counter and the 74HCT5555 also with a 24-stage counter. Linear Tech has a programmable range, the LTC6930 that will go from 32KHz to 8MHz based on a jumper code. Intersil produces the ICM7242 which will operate from microseconds to days. Another recent announced addition to Linear Tech's stable is the LTC2956 Wake-Up Timer that can stretch the timing function to 39 days between pulses and can adjust the on time as well.

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