Data Acquisition

In a typical stand-alone data acquisition application, sensor readings are taken atperiodic intervals, allowing the system to return to the standby mode conserving power,during these periods of inactivity. For example, sensor readings may only be requiredonce every 500 milliseconds. The RTC would, therefore, be programmed for an alarmwake-up event every 500 milliseconds (see low power mode). The system activity couldbe reduced to approximately 10 milliseconds in every 500 milliseconds, providing asubstantial power reduction, which is very important for battery-operated systems.The start, stop, and parity bits used for checking data integrity in asynchronoustransmission are physically generated by the universal asynchronous receiver/transmitter(UART), located between the microprocessor bus and the line driver, which interfaces tothe actual communications link.The main purpose of the UART is to look after all the routine housekeeping mattersassociated with interfacing between the parallel bus to the microprocessor and the serialcommunications link to the host computer.When transmitting, the UART performs a number of functions. These are to:• Set the correct baud rate for transmission• Interface to the microprocessor data bus and accept characters one at a time• Generate a start bit for each character• Add the data bits in a serial stream• Calculate and add the parity bit to the data stream• Terminate the serial group with the required stop bit(s)• Advise the microprocessor it is ready for the next characterThe receiving circuitry of the UART performs the following functions. These are to:• Set the correct baud rate for receipt of data• Synchronize the incoming data with the start bit• Read the data bits in a serial stream• Read the parity bits and confirm the parity• Read the stop bits• Transfer the character as a parallel data onto the microprocessor data bus• Interface the handshaking lines• Observe and report any errors associated with the character frame received.Typical errors that the UART can detect are:• Receiver overruns – bytes received faster than they are read• Parity errors – mismatch between parity bits and character frame• Framing error – all bits in the frame are zero or a break condition is reportedA break condition occurs when the transmitter that holds the data line is in a spaced (orpositive voltage) state for a period of time longer, than that required for a completecharacter. This is a method of getting the receiving UART to react immediately andperform some other task.