178EMBEDDED CONTROLLER

Hardware Design

the memory, at a reduced bandwidth. As a result, the CPU may be a bit slower because it will sometimes have to wait for a DMA cycle to complete, but it is not entirely shut out when a DMA transfer is in progress. When a single cycle DMA transfer occurs, more time is used in acquiring and transferring control of the memory to and from the DMA controller since it happens so much more fre­ quently. This “overhead” frequently reduces the overall available memory band­ width, especially when it is performed sequentially with the data transfers. Some systems overlap the memory bus arbitration handshaking with the memory data transfers so that the arbitration does not slow down the data transfers.

Direct memory access is required when the CPU is too slow to transfer the data under program control. Because the CPU does not have to participate directly in the item-by-item transfer of data, DMA is also useful when there are other tasks that the CPU can perform. In those cases, DMA transfers may be used even though they are not strictly required by the data rate.

Elementary I/O Devices and Applications

Parallel ports are the simplest form of I/O, but there are many different types of electrical interfaces ranging from the simple open collector TTL outputs used on a PC printer port to high-speed peripheral interfaces such as the IEEE-488 and SCSI buses. Most embedded controller ICs have some pins that are config­ urable as parallel input or output. These interfaces are appropriate for simple I/O, such as key switch and display interfacing. They are also appropriate for controlling and monitoring high-level interfaces such as solid-state relays.

The parallel I/O ports available on the 8051 family and similar processors are fairly versatile, with special internal circuitry to allow a port bit to be config­ ured individually as an input or output. Some microcontrollers also provide considerable current source and sink capability, however the 8051 family parts are usually fairly weak in that regard.

Serial ports, also referred to as asynchronous or synchronous communications (COM) interfaces, are commonly used to interconnect with devices, such as modems, which inherently transmit the data one bit at a time over a commu­ nication link such as a phone line. The RS-232 serial interface used in a PC’s COM port is an asynchronous serial data stream. An asynchronous interface

179CHAPTER EIGHT

Basic I/O Interfaces

has no explicit clock signal to synchronize the transfer of data. The timing of bits is based on the absolute bit rate, and is synchronized on every character with a start bit. The serial to parallel conversion is performed by a UART (universal asynchronous receiver-transmitter). When transmitting data, the UART appends a start bit before the data, shifts out the data LSB first, and adds a stop bit after the data. Once the transmission is complete, the UART sets a status bit indicating that the data has been sent and that it is ready to begin transmission of another character.

When receiving data, the UART looks for and synchronizes to the leading edge of a start bit. Then, it delays for one and one-half of a bit period, so that it samples the LSB in the middle of the bit period. Then, the UART delays one bit period, samples the next to the LSB, etc. until all the bits have been shifted in. Once all the data has been received, it is loaded into a buffer register and a status bit is set to indicate that the receive buffer contains a character and may be read by the CPU. In order for this method to work, the two UARTS at each end of the communication must have bit rate clocks that are accurate enough to guarantee that the data will be sampled at the right time. This typically requires a sample clock that is 16 times the data rate, accurate to 1% to 2%.

Timers and counters, which are present in most microcontroller chips, allow generation of pulses and interrupts at regular intervals. They can also be used to count pulses and measure event timing. Some of the more sophisticated versions can measure frequency, pulse width, and relative pulse timing on inputs. Outputs can be defined to have a given repetition rate, pulse width, and even complex sequences of pulses in some cases. In most cases, one of the timers can be used to generate the necessary serial clocks required to operate a microcontroller’s on-chip UART. In order to meet the approximately1% clock frequency accuracy for the 16x data rate clocks, the crystal frequency is often chosen to allow exact integer division of the crystal frequency resulting in an accurate, standard serial data rate. This is why 8051 family parts that use their internal counters and serial port to connect to standard 9600 bps and higher data rates use the crystal frequency 11.059 MHz rather than an even 12 MHz.

Analog to digital converters (ADCs) and digital to analog converters (DACs) are used to convert continuously variable real world parameters to digital form and back to analog. Examples include conversion of the output voltage of a temperature sensor into digital form for processing, and converting control