Interfacing the M68HC05 MCU to the MC145051 A/D Converter

Freescale Semiconductor, Inc.Application Notenc...Freescale Semiconductor, IInside theMC145051process with a graph that depicts the testing of a signal of magnitudebetween 3/8 and 4/8 of the full scale analog reference voltage. After theguesswork is done, the binary answer of 011 is written to an outputregister for further processing.In this example, the input voltage does not change over the entireconversion process. We have assumed that the signal does not changeand there is no noise to change the input voltage. In most cases asample-and-hold circuit is used to sample a voltage signal and hold it fora specific length of time until the conversion process is complete.The accuracy, linearity, and speed of the successive approximating A/Dconverter are dependent on the properties of the DAC and thecomparator. The settling time of the DAC and the speed of thecomparator determine the speed of the conversion process. Likewise, ifthe conversion demands more resolution, the time to convert will belengthened. The DAC's non-linearity will result in non-linearities withinthe ADC. All of these factors affect the digital output result of the ADC.As stated earlier, the MC145051 will convert one of the 11 analog inputsinto a 10-bit digital representation of the analog signal. The 10-bit digitalvalue is transmitted to the outside world via a serial bus. Figure 3 showsa block diagram of the MC145051. The sequence of starting aconversion, converting the voltage, and transmitting the result is:1. The CS signal is driven low to initialize the serial port that a 4-bitaddress is going to be received and the previous 10-bit digitalresult will be transmitted.2. After the 4-bit mux address is received in the mux addressregister, one of the analog inputs is selected from the analogmultiplexer. This signal is sent to the sample-and-hold to start the10-bit conversion process.3. While the 4-bit address is received, the 10-bit previouslyconverted value is sent out on the D OUT pin.4. The internal clock drives the digital control circuitry, which in turnmanipulates the successive approximation register until the 10-bitconversion is complete.AN1228 — Rev. 2.04 MOTOROLAFor More Information On This Product,Go to: www.freescale.com