Temperature Sensing and Display
| The engineers need a temperature-sensing technology that's inexpensive, accurate and covers an extensive temperature range. They've found the National Semiconductor LM70 SPI digital temperature sensor (www.national.com). For the outdoor temperature display, they've decided to use the Philips Semiconductor I2C SAA1064 four-digit seven-segment LED display driver (www.semiconductors.philips.com). National Semiconductor's LM70 digital temperature sensor offers 0.25°C resolution over a temperature range of 55°C to +150°C (that is, 67°F to +302°F). The LM70 operates with a supply voltage between +2.65V and +5.5V and consumes 490mA with a temperature accuracy of ±2°C from 40°C to +85°C. It interfaces to microprocessors through an SPI interface. The LM70 is ideal for Project Trailblazer, and each one costs about $1. Philips Semiconductor specifically designed the SAA1064 with an I2C bus interface capable of driving four seven-segment LED displays. The I2C serial interface permits connection of up to four SAA1064s to a single microprocessor synchronous serial port. One target board connected to four SAA1064s, each driving four seven-segment LEDs, enables display of temperatures at the top and bottom of the lift, in both Fahrenheit and Celsius. For example, a single temperature display with 16 digits can show the lift top temperature of 10°F and 12°C and the lift bottom temperature of 44°F and 6°C. Each SAA1064 costs about $4. The Project Trailblazer engineers need to connect an LM70 to the x86 parallel port and develop an SPI synchronous serial communication algorithm for a device driver. Embedded developers refer to synchronous serial communication algorithms as "bit-banging" routines. Then the engineers need to connect an LM70 to the MediaEngine's ARM processor. The ARM processor's Serial Port 4 is a synchronous serial port (SSP) that simplifies communications with external SPI devices. Next, the engineers need to connect an SAA1064 to the x86 parallel port and use pre-existing I2C device driver software. Finally, the engineers need to develop an I2C bit-banging routine for the PowerPC, to enable the RPX-CLLF to drive the temperature LED displays. In summary, this chapter outlines the connection and driver development for the devices listed in Table 10.1.
TIP The LM70 is one device within an LM temperature sensor family offered by National Semiconductor. Other temperature sensors, such as the LM78, provide additional interfacing capabilities. The LM78 has an on-chip temperature sensor, 7 analog inputs to monitor voltages, 3 tachometer inputs, a watchdog timer, an interrupt controller, and 32 bytes of RAM. See the National Semiconductor Web site (www.national.com) for more information. |
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