Why is this clock signal connected to a capacitor to gnd?





.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty{ margin-bottom:0;
}







7












$begingroup$


I am trying to understand the following circuit:



enter image description here



My problem is to understand why the CLK signal is connected to the capacitor (C7). The bottom side of C7 is connected with a resistor to GND. This means that the "plate" will (after some delay) have the same potential as GND. The upper "plate" will oscillate with the CLK signal.
The logic is connected to the bottom side, so I don't understand how the NAND Gate can ever get some other input than LOW on the bottom pin. This means that the NAND gate will never output LOW and thus the RAM never stores anything, which doesn't make sense.



The only effect the capacitor could have in my understanding is to delay and flatten changes in the clock signal, but I don't understand the use of this.



Also I don't understand the meaning of the resistor. It pushes the loading time of the capacitor, but after the bottom side is on GND potential there will be no current after that.



I hope someone can help me understand this.










share|improve this question











$endgroup$












  • $begingroup$
    This is a large schematic and is hard to read due to the scaling. Could you perhaps add an arrow or circle to show what capacitor you mean?
    $endgroup$
    – Hearth
    Apr 3 at 20:13












  • $begingroup$
    Can you show where the CLK signal originates?
    $endgroup$
    – vini_i
    Apr 3 at 20:14










  • $begingroup$
    @Hearth... Click on the schematic for a larger version.
    $endgroup$
    – BobT
    Apr 3 at 20:17










  • $begingroup$
    Since the clock is oscillating, why do you believe the bottom plate of capacitor is at steady state?
    $endgroup$
    – crasic
    Apr 3 at 20:17






  • 1




    $begingroup$
    Meh, I think it's an OK schematic. I've seen much worse. It's not "badly drawn" just because one part is suboptimal.
    $endgroup$
    – pipe
    Apr 4 at 8:30


















7












$begingroup$


I am trying to understand the following circuit:



enter image description here



My problem is to understand why the CLK signal is connected to the capacitor (C7). The bottom side of C7 is connected with a resistor to GND. This means that the "plate" will (after some delay) have the same potential as GND. The upper "plate" will oscillate with the CLK signal.
The logic is connected to the bottom side, so I don't understand how the NAND Gate can ever get some other input than LOW on the bottom pin. This means that the NAND gate will never output LOW and thus the RAM never stores anything, which doesn't make sense.



The only effect the capacitor could have in my understanding is to delay and flatten changes in the clock signal, but I don't understand the use of this.



Also I don't understand the meaning of the resistor. It pushes the loading time of the capacitor, but after the bottom side is on GND potential there will be no current after that.



I hope someone can help me understand this.










share|improve this question











$endgroup$












  • $begingroup$
    This is a large schematic and is hard to read due to the scaling. Could you perhaps add an arrow or circle to show what capacitor you mean?
    $endgroup$
    – Hearth
    Apr 3 at 20:13












  • $begingroup$
    Can you show where the CLK signal originates?
    $endgroup$
    – vini_i
    Apr 3 at 20:14










  • $begingroup$
    @Hearth... Click on the schematic for a larger version.
    $endgroup$
    – BobT
    Apr 3 at 20:17










  • $begingroup$
    Since the clock is oscillating, why do you believe the bottom plate of capacitor is at steady state?
    $endgroup$
    – crasic
    Apr 3 at 20:17






  • 1




    $begingroup$
    Meh, I think it's an OK schematic. I've seen much worse. It's not "badly drawn" just because one part is suboptimal.
    $endgroup$
    – pipe
    Apr 4 at 8:30














7












7








7


2



$begingroup$


I am trying to understand the following circuit:



enter image description here



My problem is to understand why the CLK signal is connected to the capacitor (C7). The bottom side of C7 is connected with a resistor to GND. This means that the "plate" will (after some delay) have the same potential as GND. The upper "plate" will oscillate with the CLK signal.
The logic is connected to the bottom side, so I don't understand how the NAND Gate can ever get some other input than LOW on the bottom pin. This means that the NAND gate will never output LOW and thus the RAM never stores anything, which doesn't make sense.



The only effect the capacitor could have in my understanding is to delay and flatten changes in the clock signal, but I don't understand the use of this.



Also I don't understand the meaning of the resistor. It pushes the loading time of the capacitor, but after the bottom side is on GND potential there will be no current after that.



I hope someone can help me understand this.










share|improve this question











$endgroup$




I am trying to understand the following circuit:



enter image description here



My problem is to understand why the CLK signal is connected to the capacitor (C7). The bottom side of C7 is connected with a resistor to GND. This means that the "plate" will (after some delay) have the same potential as GND. The upper "plate" will oscillate with the CLK signal.
The logic is connected to the bottom side, so I don't understand how the NAND Gate can ever get some other input than LOW on the bottom pin. This means that the NAND gate will never output LOW and thus the RAM never stores anything, which doesn't make sense.



The only effect the capacitor could have in my understanding is to delay and flatten changes in the clock signal, but I don't understand the use of this.



Also I don't understand the meaning of the resistor. It pushes the loading time of the capacitor, but after the bottom side is on GND potential there will be no current after that.



I hope someone can help me understand this.







capacitor clock ram






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited Apr 3 at 20:07









laptop2d

27.4k123785




27.4k123785










asked Apr 3 at 20:04









birdfreeyahoobirdfreeyahoo

1484




1484












  • $begingroup$
    This is a large schematic and is hard to read due to the scaling. Could you perhaps add an arrow or circle to show what capacitor you mean?
    $endgroup$
    – Hearth
    Apr 3 at 20:13












  • $begingroup$
    Can you show where the CLK signal originates?
    $endgroup$
    – vini_i
    Apr 3 at 20:14










  • $begingroup$
    @Hearth... Click on the schematic for a larger version.
    $endgroup$
    – BobT
    Apr 3 at 20:17










  • $begingroup$
    Since the clock is oscillating, why do you believe the bottom plate of capacitor is at steady state?
    $endgroup$
    – crasic
    Apr 3 at 20:17






  • 1




    $begingroup$
    Meh, I think it's an OK schematic. I've seen much worse. It's not "badly drawn" just because one part is suboptimal.
    $endgroup$
    – pipe
    Apr 4 at 8:30


















  • $begingroup$
    This is a large schematic and is hard to read due to the scaling. Could you perhaps add an arrow or circle to show what capacitor you mean?
    $endgroup$
    – Hearth
    Apr 3 at 20:13












  • $begingroup$
    Can you show where the CLK signal originates?
    $endgroup$
    – vini_i
    Apr 3 at 20:14










  • $begingroup$
    @Hearth... Click on the schematic for a larger version.
    $endgroup$
    – BobT
    Apr 3 at 20:17










  • $begingroup$
    Since the clock is oscillating, why do you believe the bottom plate of capacitor is at steady state?
    $endgroup$
    – crasic
    Apr 3 at 20:17






  • 1




    $begingroup$
    Meh, I think it's an OK schematic. I've seen much worse. It's not "badly drawn" just because one part is suboptimal.
    $endgroup$
    – pipe
    Apr 4 at 8:30
















$begingroup$
This is a large schematic and is hard to read due to the scaling. Could you perhaps add an arrow or circle to show what capacitor you mean?
$endgroup$
– Hearth
Apr 3 at 20:13






$begingroup$
This is a large schematic and is hard to read due to the scaling. Could you perhaps add an arrow or circle to show what capacitor you mean?
$endgroup$
– Hearth
Apr 3 at 20:13














$begingroup$
Can you show where the CLK signal originates?
$endgroup$
– vini_i
Apr 3 at 20:14




$begingroup$
Can you show where the CLK signal originates?
$endgroup$
– vini_i
Apr 3 at 20:14












$begingroup$
@Hearth... Click on the schematic for a larger version.
$endgroup$
– BobT
Apr 3 at 20:17




$begingroup$
@Hearth... Click on the schematic for a larger version.
$endgroup$
– BobT
Apr 3 at 20:17












$begingroup$
Since the clock is oscillating, why do you believe the bottom plate of capacitor is at steady state?
$endgroup$
– crasic
Apr 3 at 20:17




$begingroup$
Since the clock is oscillating, why do you believe the bottom plate of capacitor is at steady state?
$endgroup$
– crasic
Apr 3 at 20:17




1




1




$begingroup$
Meh, I think it's an OK schematic. I've seen much worse. It's not "badly drawn" just because one part is suboptimal.
$endgroup$
– pipe
Apr 4 at 8:30




$begingroup$
Meh, I think it's an OK schematic. I've seen much worse. It's not "badly drawn" just because one part is suboptimal.
$endgroup$
– pipe
Apr 4 at 8:30










4 Answers
4






active

oldest

votes


















11












$begingroup$

C7 and R58 form a high pass filter, also known as a differentiator.



The purpose of using a differentiator in this spot is to cause a short pulse on the rising and falling edge of the clock signal.



This diagram shows the effect of a differentiator on a square wave (which your clock will be.)



enter image description here



As you can see, it makes short pulses on the edges of the square wave.





I'm not sure why that circuit needs the short pulses instead of the square wave, though. Too many ICs I'd have to look up to figure out what is going on.






share|improve this answer









$endgroup$









  • 3




    $begingroup$
    I think it's an RX edge latch
    $endgroup$
    – crasic
    Apr 3 at 22:48










  • $begingroup$
    I also ask myself why this is needed. The multiplexer also allows a button push which won't be much faster then the unmodified clock signal.
    $endgroup$
    – birdfreeyahoo
    Apr 4 at 21:42



















12












$begingroup$

The circuit overall is a 16-byte memory bank for a homebrew computer of some sort, with manual programming capability via the switches and lights. Probably the most complicated 16 bytes of memory you'll ever see!



The R-C combination being asked about is used only when the CPU is running — i.e., when PROG is not asserted. It is probably being used to shorten the high time of the write-enable pulses going to the memory chips in order to meet hold-time requirements when the CPU is writing to memory. This only works if the clock high time is significantly longer than the R-C time constant.



10 nF × 1 kΩ = 10 µs



So presumably the clock is something less than 50 kHz.



Actually, the 74189 is not a slow part — the minimum write pulse width is a few tens of ns — so the R-C time constant could be much shorter, by a couple of orders of magnitude.






share|improve this answer











$endgroup$





















    1












    $begingroup$

    That is a high pass RC filter with a pole at 15.9kHz, which is unusual for a clock signal because it will attenuate the signal somewhat. The purpose is possibly to keep the clock more than 15-20kHz, to keep the memory running above that speed as a lower clock speed would not transition.






    share|improve this answer









    $endgroup$





















      0












      $begingroup$

      As others have noted, it's a differentiator that limits the time that WE is asserted.



      The outputs from the 74189s go into high impedance with WE is asserted, so it could be to make sure that the LEDs are not excessively dimmed, or to ensure that the output BUS_* lines are stable before the other edge of the clock.



      The latter option might be used of the output bus is read on the other clock edge.






      share|improve this answer









      $endgroup$














        Your Answer





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        4 Answers
        4






        active

        oldest

        votes








        4 Answers
        4






        active

        oldest

        votes









        active

        oldest

        votes






        active

        oldest

        votes









        11












        $begingroup$

        C7 and R58 form a high pass filter, also known as a differentiator.



        The purpose of using a differentiator in this spot is to cause a short pulse on the rising and falling edge of the clock signal.



        This diagram shows the effect of a differentiator on a square wave (which your clock will be.)



        enter image description here



        As you can see, it makes short pulses on the edges of the square wave.





        I'm not sure why that circuit needs the short pulses instead of the square wave, though. Too many ICs I'd have to look up to figure out what is going on.






        share|improve this answer









        $endgroup$









        • 3




          $begingroup$
          I think it's an RX edge latch
          $endgroup$
          – crasic
          Apr 3 at 22:48










        • $begingroup$
          I also ask myself why this is needed. The multiplexer also allows a button push which won't be much faster then the unmodified clock signal.
          $endgroup$
          – birdfreeyahoo
          Apr 4 at 21:42
















        11












        $begingroup$

        C7 and R58 form a high pass filter, also known as a differentiator.



        The purpose of using a differentiator in this spot is to cause a short pulse on the rising and falling edge of the clock signal.



        This diagram shows the effect of a differentiator on a square wave (which your clock will be.)



        enter image description here



        As you can see, it makes short pulses on the edges of the square wave.





        I'm not sure why that circuit needs the short pulses instead of the square wave, though. Too many ICs I'd have to look up to figure out what is going on.






        share|improve this answer









        $endgroup$









        • 3




          $begingroup$
          I think it's an RX edge latch
          $endgroup$
          – crasic
          Apr 3 at 22:48










        • $begingroup$
          I also ask myself why this is needed. The multiplexer also allows a button push which won't be much faster then the unmodified clock signal.
          $endgroup$
          – birdfreeyahoo
          Apr 4 at 21:42














        11












        11








        11





        $begingroup$

        C7 and R58 form a high pass filter, also known as a differentiator.



        The purpose of using a differentiator in this spot is to cause a short pulse on the rising and falling edge of the clock signal.



        This diagram shows the effect of a differentiator on a square wave (which your clock will be.)



        enter image description here



        As you can see, it makes short pulses on the edges of the square wave.





        I'm not sure why that circuit needs the short pulses instead of the square wave, though. Too many ICs I'd have to look up to figure out what is going on.






        share|improve this answer









        $endgroup$



        C7 and R58 form a high pass filter, also known as a differentiator.



        The purpose of using a differentiator in this spot is to cause a short pulse on the rising and falling edge of the clock signal.



        This diagram shows the effect of a differentiator on a square wave (which your clock will be.)



        enter image description here



        As you can see, it makes short pulses on the edges of the square wave.





        I'm not sure why that circuit needs the short pulses instead of the square wave, though. Too many ICs I'd have to look up to figure out what is going on.







        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered Apr 3 at 20:17









        JREJRE

        23.4k54178




        23.4k54178








        • 3




          $begingroup$
          I think it's an RX edge latch
          $endgroup$
          – crasic
          Apr 3 at 22:48










        • $begingroup$
          I also ask myself why this is needed. The multiplexer also allows a button push which won't be much faster then the unmodified clock signal.
          $endgroup$
          – birdfreeyahoo
          Apr 4 at 21:42














        • 3




          $begingroup$
          I think it's an RX edge latch
          $endgroup$
          – crasic
          Apr 3 at 22:48










        • $begingroup$
          I also ask myself why this is needed. The multiplexer also allows a button push which won't be much faster then the unmodified clock signal.
          $endgroup$
          – birdfreeyahoo
          Apr 4 at 21:42








        3




        3




        $begingroup$
        I think it's an RX edge latch
        $endgroup$
        – crasic
        Apr 3 at 22:48




        $begingroup$
        I think it's an RX edge latch
        $endgroup$
        – crasic
        Apr 3 at 22:48












        $begingroup$
        I also ask myself why this is needed. The multiplexer also allows a button push which won't be much faster then the unmodified clock signal.
        $endgroup$
        – birdfreeyahoo
        Apr 4 at 21:42




        $begingroup$
        I also ask myself why this is needed. The multiplexer also allows a button push which won't be much faster then the unmodified clock signal.
        $endgroup$
        – birdfreeyahoo
        Apr 4 at 21:42













        12












        $begingroup$

        The circuit overall is a 16-byte memory bank for a homebrew computer of some sort, with manual programming capability via the switches and lights. Probably the most complicated 16 bytes of memory you'll ever see!



        The R-C combination being asked about is used only when the CPU is running — i.e., when PROG is not asserted. It is probably being used to shorten the high time of the write-enable pulses going to the memory chips in order to meet hold-time requirements when the CPU is writing to memory. This only works if the clock high time is significantly longer than the R-C time constant.



        10 nF × 1 kΩ = 10 µs



        So presumably the clock is something less than 50 kHz.



        Actually, the 74189 is not a slow part — the minimum write pulse width is a few tens of ns — so the R-C time constant could be much shorter, by a couple of orders of magnitude.






        share|improve this answer











        $endgroup$


















          12












          $begingroup$

          The circuit overall is a 16-byte memory bank for a homebrew computer of some sort, with manual programming capability via the switches and lights. Probably the most complicated 16 bytes of memory you'll ever see!



          The R-C combination being asked about is used only when the CPU is running — i.e., when PROG is not asserted. It is probably being used to shorten the high time of the write-enable pulses going to the memory chips in order to meet hold-time requirements when the CPU is writing to memory. This only works if the clock high time is significantly longer than the R-C time constant.



          10 nF × 1 kΩ = 10 µs



          So presumably the clock is something less than 50 kHz.



          Actually, the 74189 is not a slow part — the minimum write pulse width is a few tens of ns — so the R-C time constant could be much shorter, by a couple of orders of magnitude.






          share|improve this answer











          $endgroup$
















            12












            12








            12





            $begingroup$

            The circuit overall is a 16-byte memory bank for a homebrew computer of some sort, with manual programming capability via the switches and lights. Probably the most complicated 16 bytes of memory you'll ever see!



            The R-C combination being asked about is used only when the CPU is running — i.e., when PROG is not asserted. It is probably being used to shorten the high time of the write-enable pulses going to the memory chips in order to meet hold-time requirements when the CPU is writing to memory. This only works if the clock high time is significantly longer than the R-C time constant.



            10 nF × 1 kΩ = 10 µs



            So presumably the clock is something less than 50 kHz.



            Actually, the 74189 is not a slow part — the minimum write pulse width is a few tens of ns — so the R-C time constant could be much shorter, by a couple of orders of magnitude.






            share|improve this answer











            $endgroup$



            The circuit overall is a 16-byte memory bank for a homebrew computer of some sort, with manual programming capability via the switches and lights. Probably the most complicated 16 bytes of memory you'll ever see!



            The R-C combination being asked about is used only when the CPU is running — i.e., when PROG is not asserted. It is probably being used to shorten the high time of the write-enable pulses going to the memory chips in order to meet hold-time requirements when the CPU is writing to memory. This only works if the clock high time is significantly longer than the R-C time constant.



            10 nF × 1 kΩ = 10 µs



            So presumably the clock is something less than 50 kHz.



            Actually, the 74189 is not a slow part — the minimum write pulse width is a few tens of ns — so the R-C time constant could be much shorter, by a couple of orders of magnitude.







            share|improve this answer














            share|improve this answer



            share|improve this answer








            edited Apr 3 at 21:00

























            answered Apr 3 at 20:50









            Dave TweedDave Tweed

            124k10153268




            124k10153268























                1












                $begingroup$

                That is a high pass RC filter with a pole at 15.9kHz, which is unusual for a clock signal because it will attenuate the signal somewhat. The purpose is possibly to keep the clock more than 15-20kHz, to keep the memory running above that speed as a lower clock speed would not transition.






                share|improve this answer









                $endgroup$


















                  1












                  $begingroup$

                  That is a high pass RC filter with a pole at 15.9kHz, which is unusual for a clock signal because it will attenuate the signal somewhat. The purpose is possibly to keep the clock more than 15-20kHz, to keep the memory running above that speed as a lower clock speed would not transition.






                  share|improve this answer









                  $endgroup$
















                    1












                    1








                    1





                    $begingroup$

                    That is a high pass RC filter with a pole at 15.9kHz, which is unusual for a clock signal because it will attenuate the signal somewhat. The purpose is possibly to keep the clock more than 15-20kHz, to keep the memory running above that speed as a lower clock speed would not transition.






                    share|improve this answer









                    $endgroup$



                    That is a high pass RC filter with a pole at 15.9kHz, which is unusual for a clock signal because it will attenuate the signal somewhat. The purpose is possibly to keep the clock more than 15-20kHz, to keep the memory running above that speed as a lower clock speed would not transition.







                    share|improve this answer












                    share|improve this answer



                    share|improve this answer










                    answered Apr 3 at 20:19









                    laptop2dlaptop2d

                    27.4k123785




                    27.4k123785























                        0












                        $begingroup$

                        As others have noted, it's a differentiator that limits the time that WE is asserted.



                        The outputs from the 74189s go into high impedance with WE is asserted, so it could be to make sure that the LEDs are not excessively dimmed, or to ensure that the output BUS_* lines are stable before the other edge of the clock.



                        The latter option might be used of the output bus is read on the other clock edge.






                        share|improve this answer









                        $endgroup$


















                          0












                          $begingroup$

                          As others have noted, it's a differentiator that limits the time that WE is asserted.



                          The outputs from the 74189s go into high impedance with WE is asserted, so it could be to make sure that the LEDs are not excessively dimmed, or to ensure that the output BUS_* lines are stable before the other edge of the clock.



                          The latter option might be used of the output bus is read on the other clock edge.






                          share|improve this answer









                          $endgroup$
















                            0












                            0








                            0





                            $begingroup$

                            As others have noted, it's a differentiator that limits the time that WE is asserted.



                            The outputs from the 74189s go into high impedance with WE is asserted, so it could be to make sure that the LEDs are not excessively dimmed, or to ensure that the output BUS_* lines are stable before the other edge of the clock.



                            The latter option might be used of the output bus is read on the other clock edge.






                            share|improve this answer









                            $endgroup$



                            As others have noted, it's a differentiator that limits the time that WE is asserted.



                            The outputs from the 74189s go into high impedance with WE is asserted, so it could be to make sure that the LEDs are not excessively dimmed, or to ensure that the output BUS_* lines are stable before the other edge of the clock.



                            The latter option might be used of the output bus is read on the other clock edge.







                            share|improve this answer












                            share|improve this answer



                            share|improve this answer










                            answered Apr 4 at 4:55









                            Matt TimmermansMatt Timmermans

                            1414




                            1414






























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