Electronic Design

Low Voltage Powers Parallel-Port Interface

For portable-sensing and data- acquisition applications, a laptop computer and its parallel port (LPT) make good bedfellows. Yet in the effort to extend battery life, many microprocessors and entire systems operate with logic levels down to 1.8 V. The LPT port's 5-V output signals don't easily support the low-voltage logic required to increase battery life and achieve longer data-acquisition times.

Because dynamic power consumption for a microprocessor or clocked circuit is predominately a function of voltage (P = CV2f), logic systems that operate off the parallel port at below 5 V can substantially conserve power. The circuit shown in the figure accepts data from the LPT port and delivers up to 100 mA at 2 V. Efficiency is as high as 94%. Translating 5-V logic levels at the LPT-port data pins down to 2 V, the circuit also offers ESD protection.

The low-voltage surface-mount IC labeled U1—a step-down dc-dc converter with built-in MOSFETs and a synchronous rectifier—forms a simple and highly efficient 2-V source. Feedback resistors R1 and R2 set the output voltage as low as 1.25 V. (Voltage is set at 2 V in the example circuit.) U1's shutdown pin connects directly to bit D1 of the 5-V LPT control port (pin 14), letting the device be enabled and disabled with software.

When enabled, U1's soft-start capability limits the inrush current. Schottky diodes D1 through D3 act in conjunction with pins 5 through 9 of the 5-V LPT data port to provide power for the converter. Software also easily enables and disables the data-port pins.

As a low-voltage SIM/Smart-Card level translator, U2 translates the logic levels from 5 to 2 V. U2 also provides ESD protection to the inputs. The input side of the low-voltage translator operates at 5-V logic levels and is powered by input capacitor C1 at approximately 5 V. U2's output side is powered by the 2-V supply. Data is read into the LPT status port (pin 12) serially, and clock (CLK) and chip-select (—CS) signals are derived from the LPT data port and software.

The example data interface consists of an 8-bit parallel-load shift register (U3) and an open-drain logic gate (U4). It reads U2's I/O pin and supplies parallel data. Software in the PC toggles the CLK and —CS lines to load and shift this parallel data into the program. The C++ software (see code listing at www.elecdesign.com) is configured to convert parallel data from the shift register to a serial format. After minor modifications, however, it can be configured to support a microcontroller interface or a serial SPI, I2C, or SMBus interface.

Therefore, this circuit can send data to an SPI, I2C , or SMBus serial interface, or to an 8-bit parallel-out serial shift register such as the SN74HC164, which can operate at 2 V. Many devices available today take advantage of these power savings by operating down to 1.5 V.

To download the listings, click Download the Code.

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