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r Never design write-only registers. In an attempt to minimize the address space occupied by the in Software Generator Code-39 in Software r Never design write-only registers. In an attempt to minimize the address space occupied by the




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r Never design write-only registers. In an attempt to minimize the address space occupied by the using barcode implement for software control to generate, create barcode 3/9 image in software applications. iOS peripheral, barcode 3 of 9 for None certain designs from the pioneer days had the same address serve dual purposes. A write operation would access a command register while a read would access a status register. This kind of address multiplexing must be considered a bad habit, though, as it precludes reading back the most recent command from the I/O port, which forces the programmer to maintain an external copy of the register s content.

. r Always render the current state of data transfer operations observable from the outside world. As a bad ex Software Code39 ample, consider an IC that ingests and outputs 32 bit words in sets of four subsequent bytes without making it possible for the host computer to infer whether the current transfer refers to byte 0, 1, 2 or 3. This coerces programmers of software drivers and test engineers too into resorting to o -chip shadow counters just to keep track of a circuit s internal operation. Also, a risk of losing synchronization always remains.

. mandatory c lock optional ASIC status signals event-related informations control signals that switch as part of regular system operation reset. interrupt request to and from host computer data control chip address bus lines select bus status register(s). command register(s). address decoder data-related informations unidirectional and/or bidirectional data signals data register(s). configuration-related informations control sig nals to be set during system initialization that essentially remain unchanged afterwards payload circuitry. configuration register(s). interface circuitry Fig. 8.42 A well-organized processor bus interface. Example Consider a 3 of 9 for None controller for some optical disk drive. A variety of information gets exchanged between the controller and its host. This information can be structured as follows.

. Design of VLSI Circuits Type Status Command Data Con guration Information barcode 3 of 9 for None transfer nished, data ready/needed for transfer, disk at speed, disk write-protected, head over track #, error ags, etc. spin up!, spin down!, eject disk!, search track #!, read sector!, write sector!, etc. data word read from disk, data word to be written to disk.

number of tracks, number of sectors, recording format, etc.. Table entry transfer nished is an event that would typically be handled by an interrupt, in the occurrence, whereas direct memory access (DMA) transfers are more appropriate to handle data ready/needed for transfer requests since any reaction to such events must occur within a few microseconds to prevent any data losses.. 8.5.5 Mechanical contacts Signals tha t emanate from mechanical contacts almost always exhibit spurious pulse trains instead of clear-cut edges. This is because mechanical contacts tend to recoil a couple times before closing for good. The process of breaking a contact is not clean either.

Contact bouncing typically extends over a period of 4 to 20 ms and calls for special precautions to suppress or neutralize unwanted signal transitions. The traditional approach to debouncing consisted in ltering out spurious signals by way of an SR-seesaw in conjunction with a double-throw switch (with standard break-before-make contacts), see g.8.

43a. The circuit works on the grounds that the memory loop preserves its previous state whenever the contact is broken. This continues until the blade has travelled all the way to the opposite side and has made a rst and most likely ephemeral contact there, in which case the output ips.

Figures 8.43b and c show variations on the theme. Debouncing is indeed one of the rare occasions where a zero-latency loop nds a safe and useful application.

The problem with all these approaches is that they are incompatible with the level shifters that are normally inserted between a chip s pads and its core circuitry. Figure 8.43d shows a totally di erent solution that uses a Schmitt trigger and a very low sampling rate to suppress consecutive spurious transitions.

Note that it does not matter whether a signal in transit gets interpreted as logic 0 or 1. Another bene t over gs.8.

43a and b is that this design makes do with a single-throw switch and a simpli ed wiring. What s more, this kind of debouncing lends itself well to implementation in software, thereby doing away with the necessity for any special circuitry except for a static pull-up and a level shifter, preferably with hysteresis..

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