50 Hz interrupt generator module

[Steve Cousins]

As far as I'm aware there isn't a 'standard' 50 Hz interrupt source in the RCBus world. We have the Z80 CTC but these are now difficult to get hold of and are not part of typical RCBus systems.

In an attempt to fill the gap I've created SC737.

[Alan Cox]

Nice. That's a useful gap filler.

I'd have probably chosen a 74HC4060 8)

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[Wayne Warthen]

Excellent Steve!

I have been doing some initial testing/cleanup for the new RomWBW release and was lamenting my inability to test with a simple timer interrupt.  I will definitely add support to RomWBW.

Thanks, Wayne

[Steve Cousins]

Thanks Alan

Yes, I considered the 4060 but I don't use it in any of my other projects, so using here would mean yet another component to stock and manage. Using the 393 is not as efficient but I already use them in several kits. Design is always a compromise :)

[Steve Cousins]

Here is my simple test code

                    0001  ; Interrupt module (SC737) test for SCM
                    0002  
8000:               0003  CODE        .EQU  0x8000        ;Program code starts here
8100:               0004  DATA        .EQU  0x8100        ;Data in RAM starts here
000F:               0005  Port:       .EQU  0x0F          ;Interrupt controller port
                    0006  
                    0007              .PROC Z80           ;Assemble Z80 code
                    0008  
                    0009              .ORG  CODE          ;Start of code
                    0010  
                    0011  
                    0012  ; Main program sets up 50 Hz interrupt
                    0013  
8000: 3E 07         0014  Start:      LD   A,7            ;Jump table entry 7 (mode 1 int)
8002: 11 10 80      0015              LD   DE,Int         ;Address of Interrupt handler
8005: 0E 09         0016              LD   C,9            ;API 9 = Claim jump table entry
8007: F7            0017              RST  $30            ;Claim mode 1 interrupt
8008: ED 56         0018              IM   1              ;Set mode 1 interrupt
800A: FB            0019              EI                  ;Enable interrupts
800B: 3E 80         0020              LD   A,0x80
800D: D3 0F         0021              OUT  (Port),A       ;Enable 50Hz interrupt
800F: C9            0022              RET
                    0023  
                    0024  
                    0025  ; Interrupt handler (mode 1)
                    0026  
8010: F5            0027  Int:        PUSH AF
8011: DB 0F         0028              IN   A,(Port)       ;Bit 7 low if interrupt
8013: 17            0029              RLA                 ;  is being requested
8014: 30 04         0030              JR   NC,Int50Hz     ;Interrupt requested
8016: F1            0031              POP  AF             ;  so go handle it
8017: FB            0032              EI
8018: ED 4D         0033              RETI
                    0034  
801A: D5            0035  Int50Hz:    PUSH DE
801B: AF            0036              XOR  A
801C: D3 0F         0037              OUT  (Port),A       ;Clear interrupt
801E: 3D            0038              DEC  A
801F: D3 0F         0039              OUT  (Port),A       ;Reenable interrupt
8021: ED 5B 00 81   0040              LD   DE,(Count)
8025: 13            0041              INC  DE             ;Increment counter
8026: ED 53 00 81   0042              LD   (Count),DE
802A: D1            0043              POP  DE
802B: F1            0044              POP  AF
802C: FB            0045              EI
802D: ED 4D         0046              RETI
                    0047  
                    0048              .ORG  DATA          ;Start of data in RAM
                    0049  
8100: 00 00         0050  Count:      .DW   0             ;Interrupt counter
                    0051  
                    0052  
 

[Alan Cox]

Added to EmulatorKit (option '-5')

Will take a look at adding it to the Fuzix int source detection when I have a bit more time

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[Steve Cousins]

Wow, that was quick

[Wayne Warthen]

On Thu, Jan 29, 2026 at 10:41 AM Alan Cox <etched...@gmail.com> wrote:

Added to EmulatorKit (option '-5')

Nice.  This should allow me to add support to RomWBW prior to getting the hardware.

-Wayne 

[Pellatonian]

Would substituting a 3.93216 mhz oscillator give a 60 hz interrupt?

[Steve Cousins]

Yes, a 3.93216 MHz crystal would give a 60 Hz interrupt. You may need to use different load capacitors to match the crystal.

During testing I tried running the module with a 7.3728 MHz crystal and got the expected 112.5 Hz interrupt.

[Harry Speer]

Please forgive my ignorance but  (1) Would 50 Hz be used in Europe and 60 Hz in the USA ??   (2)   I have heard of maintaining a "ticker" for a CPU + OS to do time-sharing to multiple programs and / or multiple users ??   and   (3)   How is this ticker useful, especially if you have a WatchDog Timer from the Power-Supervisor Chip,  and  also if you already have an RTC Chip with CR2032 Battery Backup for a Real-Time-Clock ?? 

   

  I'm sorry to be so "thick" but I am having trouble seeing how this "ticker" would be wanted with all-of-the-above other timers ??  Timing how long it takes to do ..... ??
HSS

[Alan Cox]

1. Not necessarily. Historically the interrupts on many machines were either generated by interesting circuitry off the mains input so was grid HZ, or was tied to the usual video scanning rate and generated by the video card. That of course was in turn is tied to the grid HZ in order to avoid flicker from lighting and so on. There's no specific reason though. The TRS80 for example uses something like 40Hz regardless of location, other machines used various fixed rates. The moment you have onboard video though there are advantages to having a frame rate timer tied to your video scanning.

2. That's one use yes

3. Power supervisor watchdogs generally don't provide an interval interrupt.

The RTC selected from the RC2014 (and coming from Retrobrew) is both incredibly slow to read and provides no interrupt. You can ask it for the time but you can't ask it to wake you up. That makes it no use for multi-tasking where a program might be in a busy loop and need to be task switched. There are RTCs that can provide time interval interrupts but the one used by that card unfortunately doesn't do so.

There are three different bits of time going on here

1. The ability to ask the time and date (RTC board)

2. The ability to get a regular interrupting event (Z80CTC, TMS9918 video, 50Hz card, and a few other odder devices)

3. The ability to do fine grained accurate timing of events (Z80 CTC)

#1 alone gets you time stamping on files in CP/M 3 and is fine for that job

#2 gets you multi-tasking and is really useful also for stuff like games

#3 lets you do things like fancy sound generation, industrial control type things, localised time bases and so on most of which are not that relevant to the RCbus machines.

Alan

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[Wayne Warthen]

To add a practical example to Alan's thorough reply, RomWBW will use a system tick to pace the output of audio files when played with the TUNE application.  The common .pt2 and .pt3 audio files want to have 50Hz pacing and play back much better when a 50Hz timer is available.

Thanks, Wayne

[Wayne Warthen]

Just wanted to post a quick follow-up that support for the SC737 Interrupt Generator Module has been implemented and tested on RomWBW.  Works very nicely.

This support is in the current RomWBW Beta Release v3.6.0 available on the RomWBW Releases Page.  The module is enabled with the config setting SCTIMENABLE.

Thanks, Wayne

[Steve Cousins]

Excellent. Thanks Wayne.

[Jaap van Ganswijk]

50 Hz makes me think of the 220V we have here in the Netherlands. The frequency of the 220V signal is very precize (over time), because clocks needed to rely on it. A processor has a crystal or resonator to make it run and you can derive a sense of time from that but it's not real-time. Therefore you need a Real Time Controller (RTC) but that also depends on it's own crystal or resonator or whatever and isn't real real-time. Another source is the internet where there are real time sources, Microsoft provides one. We also used the DCF-77 broadcast from Germany, like most home weather stations do now, but sometimes they are hard to receive and that isn't a big problem as long as they are received at least some of the time correctly. BTW, you shouldn't use RTC-signals to control task-switching etc.

Hey Cousins, I think I bought one or more of your PCB's. Thanks for doing all that work, almost started doing it myself.

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[Bill McMullen]

BTW, you shouldn't use RTC-signals to control task-switching etc.

Perhaps there's a language issue that needs clarification.  I spent MANY YEARS working on large plant-wide REAL-TIME process control systems.  In that context, there is priority-based task switching in order to try complete critical tasks within specified time constraints i.e. a one second PID loop.

Systems such as MP/M use TIME-SLICING so that multiple cpu-bound users get access to some machine cycles on a rotating basis.  MP/M uses a 50Hz or 60Hz clock signal (i.e. tick) to control this task switching.  Obviously this needs some kind of reliable signal generator like the SC737 card.

Both of the above dispatch algorithms are based on multi-tasking systems rather than typical single-task systems like CP/M.  A Time-Of-Day (TOD) clock could also be used by a scheduler for things like once-a-day reporting, time & date stamping etc.  Adding to the confusion, TOD clocks are sometimes called Real Time Clocks (RTC) in datasheets and some can also produce square waves at select frequencies which might be appropriate for time-slicing.

[Jaap van Ganswijk]

We seem to live in different worlds...

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