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MC908QY1CDTE Datasheet(PDF) 44 Page - Freescale Semiconductor, Inc |
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MC908QY1CDTE Datasheet(HTML) 44 Page - Freescale Semiconductor, Inc |
44 / 168 page Analog-to-Digital Converter (ADC) MC68HC908QY/QT Family Data Sheet, Rev. 6 44 Freescale Semiconductor 3.3.2 Voltage Conversion When the input voltage to the ADC equals VDD, the ADC converts the signal to $FF (full scale). If the input voltage equals VSS, the ADC converts it to $00. Input voltages between VDD and VSS are a straight-line linear conversion. All other input voltages will result in $FF if greater than VDD and $00 if less than VSS. NOTE Input voltage should not exceed the analog supply voltages. 3.3.3 Conversion Time Sixteen ADC internal clocks are required to perform one conversion. The ADC starts a conversion on the first rising edge of the ADC internal clock immediately following a write to the ADSCR. If the ADC internal clock is selected to run at 1 MHz, then one conversion will take 16 μs to complete. With a 1-MHz ADC internal clock the maximum sample rate is 62.5 kHz. 3.3.4 Continuous Conversion In the continuous conversion mode (ADCO = 1), the ADC continuously converts the selected channel filling the ADC data register (ADR) with new data after each conversion. Data from the previous conversion will be overwritten whether that data has been read or not. Conversions will continue until the ADCO bit is cleared. The COCO bit (ADSCR, $003C) is set after each conversion and will stay set until the next read of the ADC data register. When a conversion is in process and the ADSCR is written, the current conversion data should be discarded to prevent an incorrect reading. 3.3.5 Accuracy and Precision The conversion process is monotonic and has no missing codes. 3.4 Interrupts When the AIEN bit is set, the ADC module is capable of generating a central processor unit (CPU) interrupt after each ADC conversion. A CPU interrupt is generated if the COCO bit is at 0. The COCO bit is not used as a conversion complete flag when interrupts are enabled. 3.5 Low-Power Modes The following subsections describe the ADC in low-power modes. 3.5.1 Wait Mode The ADC continues normal operation during wait mode. Any enabled CPU interrupt request from the ADC can bring the microcontroller unit (MCU) out of wait mode. If the ADC is not required to bring the MCU out of wait mode, power down the ADC by setting the CH[4:0] bits in ADSCR to 1s before executing the WAIT instruction. 16 ADC Clock Cycles Conversion Time = ADC Clock Frequency Number of Bus Cycles = Conversion Time × Bus Frequency |
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