Hello, I have just completed reading C. Petzhold's "Code. The hidden language of computer hardware and software" (I think I need not tell you how great this book is). There is, however, something I do not grasp. The author explains how to build a set/reset counter based on a collection of D-type edge triggered flip-flops, and still I fail to understand how to set/reset this counter's lowest output bit (which is crucial to branching). Please, would you be kind enough as to point out my mistake, or perhaps, suggest a source for an alternative solution? Thank you very much for your time. A. Student P.S. And one more question, in case you kindly decide to put up with me. Does the clock signal have to be disconnected for this counter to be reset?
Post a schematic of the circuit in the question; on . Or, failing that, I think I could set up a small FTP dir on one of my websites, where you could upload it. Let me know. Cheers! Rich
Dear me! I have never used alt.binaries.schematics.electronic before, so I think I would just cause too much trouble asking how to do it. I am so ashamed of myself, I am sorry. .... I sure would, I am sorry ... Oh dear! I simply meet all the most wonderful people here on Usenet. THANK YOU VERY MUCH INDEED. Again, I would not know how to FTP. Shame on me. Thank you very much for the most encouraging words. You see, I have learned a very nice notation for representing the logic gates circuitry from another most kind and helpful person here on Usenet. If you do not mind, I will use it to specify the circuitry in question. (So, here goes.) This is a set/reset D-type edge triggered flip-flop (all schematics are due to Mr. C. Petzhold /Code/. The notation is due to another genius who, I think, does not wish to be mentioned. I am most greatful to/for both.) .------------. | | RESET | .---. | o-----------------o>| | | .-------->|NOR|-. | | .>| | | | | | '---' | | | .-|-------' | | | | | | | '-------. | | | .---. | | .---. SET | '-->| | | '>| | Q o-------o-|----o--->|NOR|-o----->|NOR|-o-----o | | | .->| | | .>| | | | | .---' '---' | | '---' | | | | | | .-|-------' | | | | | | | | | | | .--------' | '-------. .---. | | | | | .---. | .---. | CLK | | | | | | '->| | '-->| | | ~Q o->|NOT|-|-|--o-|--->|NOR|-o-. .->|NOR|-o-----o | | | '-------o>| | | '-|->| | '---' | | | '---' | | '---' | |.-|-------' | | || | | | || '-------. | | || .---. | | | |'-->| | | | DATA | '--->|NOR|-' | o--------|---------->| | | | '---' | '---------------------' abbreviated as: RESET.---. o--| | SET | | Q o--| n |--o CLK | | ~Q o--| |--o DATA | | o--| | | | '---' So, (for example) a 4-bit set/reset (natural binary code) counter looks like this. .-----. | o->| | |this NAND gate serves as |NAND |-o-. |the clock signal .>| | | | |generator | '-----' | | |(I have some doubts here, too) '---------' | | .-------------' | | | .-----. | | .>| | |the lowest bit of the counter o-| |NAND |-. |is simply | '>| | | |the negated clock signal | '-----' | | | | | | | .---. | .---. .---. .--->| | | .--->| | --->| | || | | | | | | | | | .---->| 1 |--.|.---->| 2 |--. .---->| 3 |--. ||| | | |||| | | | || | | | ||'-->| |-o------->| |-o------->| |-o| || | | ||||| | | || || | | || || .->| | ||||| .->| | || || .->| | || || | '---' ||||| | '---' || || | '---' || || '--------'|||| '--------'| || '--------'| || |||| | || | || |||| | || | || |||| | |'---------------o RESET NBC-3 || |||| | '----------------o SET NBC-3 || |||'------------------------------o RESET NBC-2 || ||'-------------------------------o SET NBC-2 |'---------------------------------------------o RESET NBC-1 '----------------------------------------------o SET NBC-1 || | | ?-o RESET NBC-0 .-------------------------------------------' ?-o SET NBC-0 | || | | .----------------------' | | || | | .-''--. | | | | V V V V NBC-3 NBC-2 NBC-1 NBC-0 NBC=Natural Binary Code So the counter should output: NBC-3 NBC-2 NBC-1 NBC-0 0 0 0 0 0 0 0 1 0 0 1 0 0 0 1 1 0 1 0 0 0 1 0 1 0 1 1 0 0 1 1 1 1 0 0 0 1 0 0 1 1 0 1 0 1 0 1 1 1 1 0 0 1 1 0 1 1 1 1 0 1 1 1 1 (here the counter will rewind) 0 0 0 0 0 0 0 1 0 0 1 0... I intend to use this counter as an Instruction Pointer in an 8088 circuitry I am working on. The problem lies in setting/resetting the lowest bit of this counter, i.e. the NBC-0 bit. (I have some other doubts too, the one I mention is the primary one.) I understand I am taking a lot of your time asking this question, so thank you very very much for your interest. All the best, Tom
Hello Richard, I am terribly sorry for the second schematic's misalignments. The justification I have is that I *DID* use the /Preview message/ function before I posted. It all looked fine. Perhaps you are still there so I will attempt to send a less lengthy (a three bit) version of the counter. This abstraction of the previously mentioned D-type set reset edge triggered flip-flop will hold here. RESET.---. o--| | SET | | Q o--| n |--o CLK | | ~Q o--| |--o DATA | | o--| | | | '---' So a 3-bit natural binary code counter looks like this: .-----. | o->| | |this NAND gate |NAND |-o-. |serves as .>| | | | |the clock signal | '-----' | | |generator '---------' | | .-------------' | | .-----. | | .>| | |the lowest bit o-| |NAND |-. |of the counter | '>| | | |is simply | '-----' | |the negated | | |clock signal | | | .---. | .---. .--->| | | .--->| | || | | | | | | .---->| 1 |--.|.---->| 2 |--. ||| | | |||| | | | ||'-->| |-o------->| |-o| || | | ||||| | | || || .->| | ||||| .->| | || || | '---' ||||| | '---' || || '--------'|||| '--------'| || |||| | || |||'---------------o RESET NBC-2 || ||'----------------o SET NBC-2 |'------------------------------o RESET NBC-1 '-------------------------------o SET NBC-1 || | ?-o RESET NBC-0 || | ?-o SET NBC-0 .---------------------------' | || | .------'| | | .-' | | | V V V NBC-2 NBC-1 NBC-0 NBC=Natural Binary Code The problem lies in setting/resetting the lowest bit of this counter, i.e. the NBC-0 bit. I am very sorry for the earlier slippage in accuracy. It was due to my inexpert using of the posting facilities. Thank you very much for bearing with me. All the best, Tom
Richard, Please, may I have your kind advice on what to do in case my second post's schematic becomes misaligned too. Thank you. Tom
I have a textfile which incorporates the schematic and my question. Please, would you still be interested in having a look at it (if so, where do I upload it)? I am sorry for the trouble. Thank you. Tom
Hi. This is Rich again, posting on an alias. I saw your ASCII schematic, and have your answer. The reason you can't preset the LSB to zero is that you're mistakenly calling the clock the LSB. It isn't. It's the clock. You need another flip-flop, which _can_ be reset, to give you bit 0, and run the clock at twice the frequency. The first stage of any counter is always a flip-flop of some kind, and bit 0 is clock/2. A free-running clock can't, and probably shouldn't, be reset arbitrarily. If you used D flip-flops in a chip, like the venerable 7474 (LS74, HC74, etc), the reset is independent of the clock, or asynchronous. I haven't analyzed your D-FF made of NORs yet, but that sounds like it's just up your alley. ;-) Good Luck! Rich
Good day, Rich, It is a relief to hear from you again. Actually, I thought you were put off by my incompetent using of the posting facilities. You did? Oh, thank you so much. Right. I see. Yes indeed. Brilliant idea. (My problem is that every time I run into a difficulty I do not trust my intuition and need to be told the solution by someone whose comptence in the given respect shows confidence and ease. Thank you.) Yes. I see. Surely, I will make a fool of myself now. For someone with broad knowledge the 74's would indeed be an alternative. For me, unfortunately, they are not. If I used them as means of construing the entire circuitry I would run into a lot of difficulties concerned with e.g. the maximum embedding of fuctions realized by chips concerned with the maximum current the chips can withstand while preserving their functionality. I feel I would not be able to resolve these. My first draft of the machine was on paper, then I read /Code/, and now I am implementing everything in Excel. I used to know a man as great as you. JUST in the right time he suggested me reading A. Turing's "Computing Machinery...". I will be thankful to him for the rest of my life. Thank you for your time. All the best, Tom P.S. Would you, perhaps, be kind enough as to write a line of comment on the way I am implementing my Turing Machine?
Dear Richard, Have a most enjoyable Christmas, and a truly Happy New Year. Thank very much again for your time. Tom
This is a very good point. Now I'll have to go back and get your original circuit, and lift the circuit out of the 7474 data sheet, and compare. };-> Well, I don't know what "then I read /Code/" means, but I know what Excel means; if you can do combinatorial logic to the degree that you can build a sequential element, you should be able to construct or find (!) a sequential primitive of some kind. (like =if (S(tn-1), "True","false"). In that case, I'd much rather just do a straight resettable D flip- flop as a primitive itself. You wouldn't have to string together three truth tables in a row (or circle!) just to get an edge trigger. Hope this Helps! Rich
Here's 74LS74 data sheet - the logic diagram is much more familiar than the one in that 74HC74 data sheet: http://www.ece.mtu.edu/ee/faculty/gearcher/EElabs/EE2304/74LS74_data_sheet.pdf It looks a lot more like yours! Cheers! Rich
Thank you. Please, do not worry. Yes. I am sorry. By /Code/ I mean a book by C. Petzhold "Code. The hidden language of computer hardware and software". I managed to do that. Thank you. I did that too. Thank you. Actually, you are the only person who I can discuss my work with. It helps tremendously to have someone to talk to. I greatly appreciate your kind interest. Thank you very much for your time. All the best, Tom P.S. Please, would you, perhaps, know of an internet resource contaning a technical reference for the Intel 4004 chip? I googled for it, but the result was very limited (this is the chip I am CURRENTLY attempting to build).
Tom For anyone interested, I have a complete Intel 4004 system with clock/memory and I/O. I also have all the chips, motherboard, schematics and documentation for the system. It's on an eBay auction currently. Ron
