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MC68HC908QT4 Datasheet(PDF) 3 Page - Freescale Semiconductor, Inc |
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MC68HC908QT4 Datasheet(HTML) 3 Page - Freescale Semiconductor, Inc |
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3 / 24 page  AN1950 Sensors Freescale Semiconductor 3 Figure 3. Amplification Scheme Microprocessor To provide the signal processing for pressure values, a microprocessor is needed. The MCU chosen for this application is the MC68HC908QT4. This MCU is perfect for appliance applications due to its low cost, small eight-pin package, and other on-chip resources. The MC68HC908QT4 provides: a four-channel, eight-bit A/D, a 16-bit timer, a trimmable internal timer, and in-system FLASH programming. The central processing unit is based on the high performance M68HC08 CPU core and it can address 64 Kbytes of memory space. The MC68HC908QT4 provides 4096 bytes of user FLASH and 128 bytes of random access memory (RAM) for ease of software development and maintenance. There are five bi-directional input/output lines and one input line shared with other pin features. The MCU is available in eight-pin as well as 16-pin packages in both PDIP and SOIC. For this application, the eight-pin PDIP was selected. The eight-pin PDIP was chosen for a small package, eventually to be designed into applications as the eight-pin SOIC. The PDIP enables the customer to reprogram the software on a programming board and retest. Display Depending on the quality of the display required, water level and water flow can be shown with two LEDs. If a higher quality, digital output is needed, an optional LCD interface is provided on the reference board. Using a shift register to hold display data, the LCD is driven with only three lines outputted from the microcontroller: an enable line, a data line, and a clock signal. The two LEDs are multiplexed with the data line and clock signal Figure 4. Multiplexed LCD Circuit Multiplexing of the microcontroller output pins allows communication of the LCD to be accomplished with three pins instead of eight or 11 pins of I/O lines usually needed. With an eight-bit shift register, we are able to manually clock in eight bits of data. The enable line (EN) is manually accepted when eight bytes have been shifted in, telling the LCD the data on the data bus is available to execute. The LEDs are used to show pressure output data by displaying binary values corresponding to a pressure range. Leak detection, or water-flow speed, is displayed by blinking a green LED at a speed relating to the speed of water flow. The red LED displays the direction of water flow. Turning the red LED off signifies water flowing into the tub. Turning the red LED on signifies water flowing out of the tub, or alternatively, there is a leak. Digital values for water height, rate of water flow, and calibration values are displayed if an LCD is connected to the board R5 10K R7 10Ω R8 10Ω R6 10K R11 1K R9 1K R10 500K R12 500K V2sensor V4sensor VOUT VCC C5 0.1µF 6 5 7 2 3 1 9 10 8 13 12 14 11 4 VCC VA VC VB + - + - + - + - HC908QT4 HC164 LCD EN RS RW DB0 DB1 DB2 DB3 DB4 DB5 DB6 DB7 A B CLK PTA4 PTA3 PTA5 R3 R2 1K 1K |
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