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The HC11 datasheet indicates that the minimum high voltage (VIH) for any digital input pin (excluding RESET) is 3.5VDC (0.7 X VDD), but I have seen many designs that use 3.3VDC devices (FPGAs, etc.) driving HC11 input pins with no pullups to 5V that work fine. Has anyone tested to see how low the input voltage on an HC11 digital input pin can actually go and still be interpreted by the device as a logic "1"? If so, is this documented anywhere? Thanx. |
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MC68HC11 Digital Input High Voltage Minimum
Started by ●March 5, 2004
Reply by ●March 5, 20042004-03-05
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In my opinion, you're in a gray area here. If you ask Motorola for
an official opinion they'll tell you to go by the data sheet, and provide 70% of VCC. If you release a design to production that provides outputs from 3.3 VDC to a 68HC11 run at 5 VDC, you could get away with it. On the other hand, you could have intermittent flakey operation. Things to worry about- What happens over the full operating temperature range of your equipment? When you decide to cut corners on a manufacturer's data sheet, you now have the responsibility to test all the corners yourself-power supply voltages, temperatures, a full range of operating conditions. What about power supply tolerances? If your 3.3 VDC supply is 5% low, its outputs will be at the most about 3.14 VDC. If at the same time your 5 VDC supply is 5% high, or 5.25 VDC, then the 70% input voltage requirement is 3.68 VDC, so the provided inputs will only be about 60% of the 68HC11's supply rail, or about 15% below the specified 70% level. What about noise immunity? If you drive a 5V chip from a 5V CMOS output, you have as much as 30% of the power rail, or around 1.5 VDC, before a noisy signal falls into the gray area. If you drive the 68HC11 from a 3.3 VDC circuit, you have very little noise immunity, and even small glitches on the 3.3 VDC signal will fall into the gray area. If the number of circuits between the 3.3 VDC logic, and the 68HC11 inputs, is reasonable, you could use a chip like a 74HCT244, powered from 5 VDC, but with 2.0 V input thresholds, and convert the signal levels properly from 3.3 VDC to 5 VDC. These parts are not expensive, though they do take up board space. I wouldn't ship a design with these issues unresolved. Couldn't sleep at night. If the product failed in the field, would have no ready excuse. Best regards, Kerry Berland Silicon Engines 2101 Oxford Road 847-803-6860 Fax 847-803-6870 Des Plaines, IL 60018 USA -----Original Message----- From: bengalsaint [mailto:] Sent: Friday, March 05, 2004 1:36 PM To: Subject: [m68HC11] MC68HC11 Digital Input High Voltage Minimum The HC11 datasheet indicates that the minimum high voltage (VIH) for any digital input pin (excluding RESET) is 3.5VDC (0.7 X VDD), but I have seen many designs that use 3.3VDC devices (FPGAs, etc.) driving HC11 input pins with no pullups to 5V that work fine. Has anyone tested to see how low the input voltage on an HC11 digital input pin can actually go and still be interpreted by the device as a logic "1"? If so, is this documented anywhere? Thanx. _____ > Service. |
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Reply by ●March 6, 20042004-03-06
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There are several manufacturers that make bi-directional level-shifting interface chips. Basically they have two VCC pins, one for the 5V side, one for the lower voltage side. Then they give you two pins for each IO connection. One pin of the pair connects to the high voltage side, one to the low-voltage side. One example would be the Fairchild FSTD16450. There are many more, and you should be able to find something with approximately the same number of IO channels as you need without having to go too far over. __________________________________ |
