Pulse counter

15 August 2023
A simple circuit that counts incoming pulses and displays the number of rising or falling edges detected on an LED display. Based around a Microchip PIC16F620 it is intended for use in a future project which will involve emitting pulse trains of specific length although it was also an excuse to use up some old stock of components of which most were likely sourced six years ago. This was supposed to be a short and simple project but it got plagued by faulty components so ended up taking almost two months to complete, but in the end it worked perfectly without any circuit rework.

Physical circuit

Circuit design

Unusually this circuit was designed entirely in gpsim and the firmware implemented before any hardware work was done, which was due to the project being conceived while on the road mid-June and the firmware itself as ready as it could possibly be without actual hardware by early July. The NOT-gates are used as stand-ins for transistors although an alternative would have been to sink current directly via a microcontroller pin and tristating the pin by switching between input and output modes in order to avoid excessive reverse-voltages across LED segments, although this would have complicated probe-testing of the hardware circuit prior to installing the microcontroller chip.

Circuit in gpsim

The firmware required minimal changes when hardware became available to run it, and with this expectation the microcontroller was flashed using a chip programming seat rather than having an in-circuit programming header. Since the thermocouple control was a lockdown-era project for which my recollection was having to use a make-shift rig on solderless breadboard, the last time I used this programming seat would have been late-2018 for the 4-digit 17-segment display. In situations where space is at such a premium that a flash header cannot be included the preferable solution is to use a SOIC chip flashed using a clip but this has its own set of problems. Since I no longer use PIC16F88 chips, the DIP packaged microcontrollers I typically stock these days are all fourteen pins (e.g PIC16F1823 and PIC16F1454 ) or less so I am considering making myself a new programming seat with a smaller socket and a built-in power supply.

Programming seat

The PIC16F630 chip

The Microchip PIC16F620 was the first microcontroller I ever tried using back in 2017 and in hindsight this was probably due to it being a low-cost part easily obtained from E-Bay. It was a good chip to start with as people starting out are typically only interested in basic digital I/O and this chip has little else in terms of options to trip up the inexperienced, having just the single fixed clock speed and most pins not having secondary functions in addition to bit-banging, and at least for the general-purpose registers there is no need to care about memory bank switching. It does have its issues though and the last time I used one was back in 2018.

Reliability issyes

In total I got through three PIC16F620 chips by the time the circuit was done and dusted which reminded me of problems I had in my early days of firmware writing back in 2017. Now some of this may well be down to mistakes on my part but suspicions are that the PIC16F620 has been widely cloned and the units obtained from E-Bay had poor-quality program flash memory if not outright fakes. I do wonder whether some of my earliest problems were due to using knock-off parts rather than any major fault on my part. The first was practically dead-on-arrival and the second could initally be flashed but it was throwing up checking and validation errors and it too eventually stopped working completely. The two dead chips are shown below

Dead PIC16F630 chips

The option register

What really gives the game away with regards to the thinking behind the design of the PIC16F630 is the single data-sheet page that documents the OPTION_REG register, and it certainly hints that it dates from a time when expandability was at best an after-thought. In total it affects four different sets of functionality which is a lot to squeeze into eight bits and these systems are things that are fully independent on other PIC microcontrollers I have commonly used whereas here they are not, and their implementation has introduced subtle inter-dependencies which thankfully I have been able to avoid. Thee four pieces of functionality are the following: The main thing that stuck out was the chip having a single pre-scaler that has to be assigned to either Timer0 or the watchdog timer, which suggests the original design dates back to when even the transistor count of a pre-scaler was significant. This is just one of many indications that this chip was originally designed to a very tight budget.


It is not clear why but the PIC16F620 has two different interrupt-on-change modes — one specific to the pin RA2 on Port A and another that covers all pins on Port A — but that is what it has and I expect that it is another sign of a chip made to a tight budget. The first of these is a Schmitt-triggered edge detector although unlike the PIC16F182x it can only detect a rising or falling edge and not both art the same time, whereas the interrupt-on-change that covers the whole port instead uses some mechanisms where the pins are sampled and this is compared with the internal latches. Only the latter one specific to RA2 is used and this pin is the input pulse train that is to be counted.

Instruction set

While the majority of the Microchip PIC microcontrollers' instruction set is common across at least all the ones I have used, the PIC16F630 omits several instructions that the PIC16F182x family I have normally used has. One instruction I really missed was BRW which can be used to implement jump tables rather than resorting to if-then-else chains which in assembly gets messy, so the LED segnment lighting code turned into a load of cut'n'paste which is a magnet for bugs. While the XXX does include an indirection register which is basically a form of pointer it is a pain having only one rather than two, and it does not include instructions the later chipsets have which are intended to aid their use. For a task like this it would be nice to have a single eight-pin port controlling all the LED segments, since then setting the display would be a simple lookup and writing task, but the PIC microcontrollers that do have such ports are the larger ones that have three or more I/O ports.

Timer0 accuracy

I have had long-standing doubts about how accurate PIC timers are and even if the data-sheets correctly describes them and finally decided it was time to actually check whether my suspicions were well-founded or not. To do this the following simple oscillation firmware was used, which with a prescale of 1:256 and a full 8 bits of TMR0 before the wrap-around interrupt is asserted should give a delay of 65.536mS:


Under simulation this stabilised at 65.537mS but on an actual chip hooked up to an oscilloscope it was giving 56mS which is vaguely ball-park but the 15% deviation makes it more or less useless for actual timing purposes. I have no idea if this chip was from Farnell or E-Bay but I suspect it is likley the latter as I doubt that a genuine part would be that much out of specification.

List of components

The choice of components was biased towards stock that I was pretty certain dated back to 2016, although exclusively using wire from the single reel obtained from Maplin in those early days was perhaps a little dogmatic. Strangely enough the resisitors have actually gone down in price despite inflation and I would not be surprised if one or two others have as well. The one anomaly in this list is the use of MOSFETs for sinking the LED display current as these are the one component that I had no experience with prior until 2021 although I would have taken to them fairly quickly as their high input impedance negates any needs for protective resistors.

Description Manufacturer Part number
Microcontroller Microchip PIC16F630
14-pin DIP socket Amphenol DILB14P-223TLF
7-segment LED display Multicomp LS0565SRWK
390Ω resistors TE Connectivity CFR25J390R
Perfboard Multicomp MC01008
N-type MOSFET Microchip TN5325N3-G
22AWG copper wire Not known
Tactile button TE Connectivity FSM4JH


It was only a school-boy error of not zeroing a variable on startup that prevented the firmware working the first time it was flashed to hardware, and looking back this slipped through since a significant portion of brain-power was being diverted towards figuring out a relatively restricted instruction set that I had not used for years. Aside from delays caused by faulty chips this was a nice little project that would have otherwise nicely filled in a single sitting, and there is of course that sentimental thing about going back to things that have not been used for such a long time.