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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