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FIGURE 7.27 USBN9602 Interface to AT90S8515 AVR Controller.
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The IrDA link works by sending an encoded data stream serially using an infrared LED. For receiving, it has an infrared receiver diode which feeds a decoder and a serial-to-parallel converter. Figure 7.28 illustrates the physical part of the IrDA data link. IrDA allows data transfer speeds between 2.4 Kbits/s to 4 Mbits/s. Link speed at startup is always at 9600 bits/s and then can be negotiated to a higher or lower mutually acceptable speed by the transmitter and receiver. Up to the data transfer speeds of 115 Kbits/s, the IrDA encodes the incoming serial data (from the parallel-to-serial converter) using a Return to Zero Inverted (RZI) scheme. This is the version 1.0 mode of communication also called SIR (Serial InfraRead). Above speeds of 115 Kbits/s and up to 4 Mbits/s, it is Version 1.1 and is called FIR (Fast InfraRed). FIR specifies a total of three data transfer speeds: 0.576 Mbits/s, 1.152 Mbits/s, and 4 Mbits/s. Data transfer speeds of 0.576 Mbits/s and 1.152 Mbits/s use an HDLC type of encoding scheme. At 4 Mbits/s, the encoding is done using a Pulse Positioning Modulation (PPM) scheme. However, like SIR, the initial startup operating speed is 9600 bits/s and in SIR mode. Later, on mutual agreement, the devices could move to higher speeds. The RZI encoding scheme sends a short optical pulse for every logic 0 for a short duration and no pulse for a logic 1. The duration of the optical pulse is 3/16-bit time or 1.6 s (which is 3/16 of bit time at 115 Kbits/s) (Figure 7.29). To equip an AVR device with IrDA capability, one needs an IrDA encoder, IR driver, receiver, and IR LED and diode, as illustrated in Figure 7.30 connected to the integrated serial port available in most AVR processors. The MAX3100 universal asynchronous receiver transmitter (UART) is specifically optimized for small microcontroller-based systems. It is suitable for connecting an AVR processor to MAX3100 using an SPI/Microwire interface. The MAX3100 has an IrDA SIR Timing Compatable, and it only needs an external LED driver and receiver to make a complete IrDA link, as illustrated in Figure 7.31.
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FIGURE 7.28 IrDA physical layer block diagram.
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Stop Bit
Serial Data
IR Pulse
bit time 3/16 bit time
FIGURE 7.29 RZI data encoding scheme employed by IrDA data link.
AVR IR LED IrDA Encoder TxD LED Driver
IR DIODE IrDA Decoder RxD Pre-amp and Quantizer
FIGURE 7.30 An AVR processor with an IrDA data link.
7.11 CAN (Controller Area Network) Bus
The CAN bus is a serial communication link used in automobiles as a means of communicating between the various controllers used inside cars and other vehicles. As we mentioned, a modern car has over 50 processors. The CAN bus allows all these processors to talk to each other on a single link, thus reducing cable length, which would otherwise get out of control. Without the CAN bus, the cable length in a modern car could be as much as a few thousand meters. The CAN bus was designed by a German company, Bosch, originally for automotive use. However, it is being also used as a general-purpose industrial bus (Figure 7.32). The CAN bus was designed with the following needs in mind:
MAX3100 AVR IR LED LED Driver Din Port Pins Dout SCLK CS* Rx Pre-amp and Quantizer IR DIODE Tx
FIGURE 7.31 An AVR processor interface to MAX3100.
Device 1 CANH
Device 2
CAN Bus Vd Termination Resistor Device 3 Device 4 CANL Termination Resistor
FIGURE 7.32 CAN bus topology and signals. 1. 2. 3. 4.
Support low- to high-speed data transfer rates. Provide error-free data transfer. Allow variable data volume transfers. Offers ease of maintenance and low cost.
Philips has introduced a stand-alone CAN controller chip SJA1000, which can in principle be interfaced to an AVR device.
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ill now we have considered the internals of the AVR processor in some detail, as well as looked at the ways the AVR processor can be used to connect to the external world. However, the big question is how to get the AVR processor to actually do something that you want. How do you develop the application and get the system working What are the tools available to complete the design on time and within budget This chapter will answer some of these questions. What are the steps in creating a successful system We have briefly looked at this question in a previous chapter. We need to define the design requirements to begin with. Then we need to identify the hardware that will satisfy these needs. Since we are dealing with the AVR controllers, we assume that you have zeroed in on a particular AVR controller out of the many AVR controllers listed in Table 3.11. Once you have chosen the controller, you need to understand how to write the code for the AVR controller, how is it tested, and how it is eventually loaded in the system.
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