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Figure 6.39 ULN2003A drivers used to drive inductive loads.
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4 MHz 22 pF 22 pF
Figure 6.40 A stepper motor sequencer and driver interface to AVR.
The EEPROM has a write protect pin (WP) that can be connected to 5 V to disable any write to the EEPROM. For our use, we have connected it to ground so that we can write data to the EEPROM. The EEPROM we have chosen is AT24C512 from Atmel. It has a capacity of 64 Kbytes. The EEPROM has two device address lines that allow up to four such EEPROM chips to be connected to the same IIC bus (Figure 6.43).
Figure 6.41 Circuit schematic for a stepper motor sequencer and driver for the AVR processors.
144 HARDWARE AND SOFTWARE INTERFACING WITH THE AVR
Fmax Frequency Fmin
Time
Figure 6.42 Ramping the stepper motor speed.
R/W*
Figure 6.43 EEPROM device address.
The EEPROM can be read and written in the following ways:
1. Byte Write: Figure 6.44 illustrates a byte write to the EEPROM. Following the Start
sequence, the device address is transmitted with the R/W bit reset to 0 , followed by the address of the location to which the byte is to be written to. Since the capacity of the EEPROM is 64 Kbytes, a 16-bit address split as MSB address and LSB address is sent next in that order. In the end, the data byte to be written is sent. The sequence is terminated with a Stop sequence. 2. Page Write: Same sequence as the byte write, except that multiple data bytes that are to be written are transmitted before the Stop sequence is issued. 3. Current Address Read: The Start sequence is issued followed by the device address (R/W bit is set to 1 ) and the data byte from the EEPROM is received. Figure 6.45 illustrates the transfer. 4. Random and Sequential Read: This requires a dummy write sequence to precede the actual read. The purpose is to provide the address from where to read the data. The Start sequence is issued followed by a device address that includes the R/W bit reset to 0 to indicate write. Then follows the MSB address and LSB address, and after that a new Start sequence is issued, followed by the device address sequence again with the R/W bit set to 1 to indicate the read operation. After this, the device provides the data from the required location and terminated with a Stop sequence. Since the initial write sequence is not terminated with a Stop sequence, the write to the location is not performed; instead, only the address gets changed. Figure 6.46 illustrates the data transfer
Ack Write MSB Address
Ack LSB Address
Ack Data
A1 A0 0
Start
Stop
Figure 6.44 EEPROM write byte.
146 HARDWARE AND SOFTWARE INTERFACING WITH THE AVR
Ack Read No Ack Data From EEPROM
A1 A0 1
Stop Start
Figure 6.45 EEPROM current address read.
sequence. If an Ack is generated by the processor before the Stop sequence, then data byte from the next location is received, leading to sequential read operation. The last data read must be terminated with a No Ack before the Stop sequence. The sample program to work with the circuit schematic in Figure 6.47 is available in the code directory in the file ep1byt.asm, which can read and write one byte at a time at the address you specify, and ep2byt.asm, which can read and write two bytes at a time at the specified address.
6.14 Interfacing to a Real Time Clock (RTC)
RTCs are useful devices as timekeepers in embedded systems. Many serial communication format RTCs in 8-pin DIP package with a host of features are available. We will interface DS1302 RTC from Dallas to AVR processors. This RTC can trickle charge an external standby NiCd battery. It contains 31 bytes of SRAM. The RTC has a simple three-wire interface to a microprocessor. The RTC provides seconds, minutes, hours, day, date, month, and year information. The RTC can operate in a 12-hour format with an AM/PM indicator or a 24-hour format. The RTC has a synchronous serial communication interface. Only three wires are required to communicate with a processor such as the AVR. Figure 6.48 illustrates a block diagram on an AVR interface to the DS1302 RTC. Figure 6.49 illustrates the circuit schematic. MAX232 has been added so that the user can communicate with the processor and read and write to the RTC. The code for this project is available in the code directory in the file rtc_ex.asm. The data sheet is available here:
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