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June 1998 |
Revision 1.01 |
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Persistor Instruments Inc. |
© 1998 All rights reserved. |
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June 1998 |
Revision 1.01 |
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Persistor Instruments Inc. |
© 1998 All rights reserved. |
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This application note exists as a starting point for CF1 owners who need additional digital I/O. This example presents a circuit design and C code for accessing your new I/O. As with any outside design, its applicability for your desired goal is not guaranteed. There are also limitations to this system which will be discussed as the note moves along. This note has been written to make use of fairly common parts, and wherever possible, we have used parts available to consumers from the DigiKey corporation. The code has been written for simplicity. This I/O is memory mapped onto /CS8 in this example but could just as easily be mapped with /CS10. With that said, let's jump right in!
We did several circuit designs that cover different bases with different levels of complexity. At this point, this document branches off into the different designs. The following list is organized from simplest to most complex, and we highly encourage you to understand each design before attempting to understand the next one. The progression is a logical one.
Bus Capacitance and Related Things...Because the CF1 is so small, its bus traces are very short. This has allowed us to keep the bus transactions very fast and minimize the number of wait states that need to be used. When you add devices to the bus, you increase the bus capacitance, which can have catastrophic effects on system performance if not managed correctly. Explaining the technical details of this problem is far beyond the scope of this manual, and you should consult an electrical engineer with experience with digital systems before adding any peripherals, if you yourself do not understand what we are talking about here. In internal development of peripherals for the CF1, we have found that PC board traces as short as a few inches in length have caused problems with increased bus capacitance. There are several things you can do to prevent these problems. First, keep your designs tight. Make sure that when putting peripherals on the bus, you keep those peripherals as close as possible to the bus connector. Even one device can make a huge difference. Second, if you are adding multiple peripherals, such as with our 32 I/O line expansion, you should consider putting a buffer on the bus between the CF1 and your peripherals so that any bus problems introduced by your peripherals do not affect the main system. Third, after adding each new peripheral, recharacterize your CF1 using the "Hurry up and wait" example to determine your new maximum clock frequency and minimum wait states. When you perform this recharacterization, you want to be sure to allow your self a lot of headroom. If the Persistor stops working at 16.5 MHz, you are too close to the limits. A good rule of thumb would be to allow 25 percent headroom, so you would want your new design to function up to 20MHz. Also if temperature is a factor in your design, in other words if your Persistor needs to be reliable at temperatures other than room temperature, it would be wise to recharacterize your CF1 over the entire temperature range in which you intend to operate.
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Tel: 508-759-6434 |
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Fax: 508-759-6436 |
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