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7.4 RS-485 Link
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The RS-485 communication also differs from the RS-232 in the way the bit logic levels are translated into line voltages, the maximum possible data rates, and the length of the cable and the direction of traffic. RS-485 is a communication bus and allows multiple devices to communicate on the link. As a consequence, it is a half-duplex link, i.e., at any given time a device
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162 COMMUNICATION LINKS FOR THE AVR PROCESSOR
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FIGURE 7.5 Connecting an AVR device to another AVR device using an RS-422 link.
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Original TTL Signal
FIGURE 7.6 Original data and the corresponding differential outputs of an RS422 driver.
on the bus can either receive data or transmit data. Figure 7.7 illustrates the way an RS-485 link for multiple AVR devices could be configured. Since at a given time, only one of the AVR processors can transmit, the rest of the AVR processors must remain as receivers. To achieve this, the RS-485 driver IC has an enable pin for the transmitter as well as the receiver. These pins are connected to other port pins of the respective AVR processor, so that under software control, the AVR can decide whether to transmit or to receive data. This puts more software overheads on the system. Also, the designer must decide beforehand which of the AVR processors is going to be the master processor at power on. After the power is applied (or after system reset), only this AVR device enables its transmitter and sends out data on the bus. The reset of the processors enables their respective receiver and receive data from the bus. The control of the bus (i.e., which AVR device gets to transmit) is subsequently transferred in software between these AVR processors as
SPI AND MICROWIRE BUS 163
AVR TxD P3.2 P3.3 RxD
AVR TxD P3.2 P3.3 RxD
RS485 Drivers
P3.3 P3.2 RxD
FIGURE 7.7 Connecting multiple AVR devices on a RS-485 bus.
desired. However, it has to be ensured by the software on all the devices that one and only one AVR transmits on the bus at a given time. For this purpose, it is usual to allocate unique device numbers to the various processors. The RS-485 data speeds can be up to a maximum of 10 Mbits/s and up to a maximum distance of 1000 meters (but not at maximum speed). It also allows up to 32 drivers and receivers on the bus. MAXIM makes RS-485 drivers such as MAX485, which is a drop-in replacement for the immensely popular LTC485 from Linear Technology.
7.5 SPI and MICROWIRE Bus
The Serial Peripheral Interface (SPI) circuit is a synchronous serial data link that is standard across many Motorola microprocessors and other peripheral chips. It has become so popular that many other controllers support it, including the AVR processors. As the name implies, the SPI is used to allow the controller to communicate with the peripheral devices. SPI peripheral devices from simple shift registers to ADCs, DACs, and memory chips are available. SPI supports a high data rate of up to 3 MHz. Controllers with integrated SPI ports are available to connect to peripheral devices with SPI ports. However, the controller can also use its ordinary I/O lines to mimic an SPI port, though at the cost of data transfer speed. The SPI port has the following signals:
1. 2. 3. 4.
MISO: Serial Data Output signal. MOSI: Serial Data Input Signal. SCK: Serial Clock. Select signal.
P3.3 P3.2 RxD
164 COMMUNICATION LINKS FOR THE AVR PROCESSOR
Many of the AVR processors are equipped with an SPI port. The lower-end devices that do not have this capability can be used to connect to SPI peripheral devices using the timing diagram illustrated in Figure 7.8. The figure illustrates the data output by the master to the slave on the MOSI line and the validity of this data with respect to the serial clock SCK signal. The other section of this diagram illustrates the way the data is read out of the slave on the MISO line by the master. In the AVR processors, the SCK clock frequency is limited by the system clock frequency and with current processors, it can go up to about 2.5 MHz. It should be noted that the data transfer in an SPI system is in multiples of 8 bits at a time. The MICROWIRE is a communication protocol similar to the SPI, but the data transfer is in chunks of 16 bits at a time. The data transfer speed is also limited to 1 MHz.
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