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This paper describes a keypad interface that frees microcontroller input/output pins. In most keypads, pressing a key closes a contact that bridges two lines in an xy matrix. The pulse's width is proportional to the number of the pressed key, and the microcontroller identifies the pressed key by measuring the pulse's width. IC2, a complementary metal oxide semiconductor LMC555 version of the popular 555 timer, operates as a monostable one-shot multivibrator. In the circuit's resting state, a transistor internal to IC2 at Pin 7 shunts C6 and the output of IC2 at Pin 3 remains at logic low. The output of IC1 goes from five to zero volt, which in turn triggers Pin 2 of IC2. The smallest relative change in pulse width occurs at the longest pulse ratio, 110/1,320 or 8. 33 percent. This ratio provides sufficient margin to allow use of standard ±one percent tolerance or better components that are ±0.5 and ±one percent. The keypad circuit's output pulse drives the external interrupt input, RA2 of a Microchip 16F630 microcontroller. As a safeguard against erroneous data, the routine returns an error code if the pulse width falls outside certain limitations.

This paper considers one-time-programmable nonvolatile-memory technology as a way to trim analog circuits. The key to keeping cost down is using a nonvolatile-memory technology that adds no masking steps during the circuit's fabrication. Even in a low-bit application, such as analog trimming, additional masking operations, such as those for thin-film resistor or erasable programmable read-only memory implementations, could significantly raise chip cost. The use of a standard complementary metal oxide semiconductor nonvolatile-memory technology also permits analog trimming to scale down as process nodes shrink, a capability that many other trimming techniques lack. An analog trimming with embedded nonvolatile memories in mixed-signal chips has another benefit: Chip designers can use the same nonvolatile-memory technology for higher-bit-count applications for the digital portions of the chip, such as for secure, small-area storage of firmware or other code that the chip's processing engines use.

This paper features the SM2064 digital multimeter from Signametrics Corp. The Signametrics SM2064 digital multimeter puts high precision into a Peripheral Component Interconnect plug-in form, well-suited for test-system integrators and instrumentation users. It features measurement rates to 20,000 readings/second without latency, the unit has resolution as low as 10 nanovolt; the lowest current measurement is 50 picoampere. In addition to basic voltage and current measurements, it can perform six-wire ohm readings, inductance and capacitance readings, frequency and timing measurements, and thermocouple/ resistance temperature detection measurements. It also functions as a voltage or current source and a pulse and pattern generator. The $2,595 unit may have more functions than one need, so Signametrics offers the basic SM2064 unit, still with the same precision specifications but 1400 readings/second, for $1,595.