Timeline for Preset logic for Flip-Flop down counters. How can I preset the initial value until the count starts?
Current License: CC BY-SA 4.0
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| Oct 21 at 10:29 | history | edited | toolic | CC BY-SA 4.0 |
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| Oct 21 at 5:58 | comment | added | periblepsis | @TomásSalvoAportone You can also use the \$\overline{Q}\$ outputs (if you have them.) For example, if you want the reset startup state to be 9 but the FFs you have are guaranteed only to present 0 at poweron, then use the \$\overline{Q}\$ outputs of the least and most significant FFs of the four FFs being used. Simple enough to do when both Q outputs are available. | |
| Oct 21 at 5:54 | comment | added | periblepsis | @SimonFitch That's why you use a 7476 in a discrete system requiring a known power-up state. (As I did when building my first computer in 1974.) | |
| Oct 21 at 5:54 | answer | added | the busybee | timeline score: 0 | |
| Oct 21 at 5:45 | comment | added | the busybee | And even a reproducible initial state of discrete flip-flops may be just a result of the specific circuit. | |
| Oct 21 at 5:28 | comment | added | Simon Fitch | Perhaps I spoke to soon, using an FPGA to implement JK flip-flops could guarantee a starting state, since the device will have an initial reset cycle, but discrete flip-flops (such as you'd find in a 7474) do not. | |
| Oct 21 at 5:24 | comment | added | Tomás Salvo Aportone | Huh, interesting. Then is my board's display showing a 0 before starting the count because there just isn't an assigned value to Y[3..0] yet? And thanks, learning about this stuff is quite interesting! | |
| Oct 21 at 5:14 | comment | added | Simon Fitch | Flip-flops don't have a default value, they can assume any value immediately after power is applied. A simulator may suggest predictability, but real flip-flops have no guaranteed start-up value. That doesn't answer your question, it's just FYI. | |
| Oct 21 at 1:58 | history | edited | Tomás Salvo Aportone | CC BY-SA 4.0 |
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| S Oct 21 at 1:56 | review | First questions | |||
| Oct 21 at 6:57 | |||||
| S Oct 21 at 1:56 | history | asked | Tomás Salvo Aportone | CC BY-SA 4.0 |