S15: RFID Security Box - Revision history

Proj user18: /* Shift register bus overview */

2015-06-03T14:51:19Z

‎Shift register bus overview

Proj user18: /* Software Design */

2015-06-03T14:18:26Z

‎Software Design

Proj user18: /* Software Design */

2015-06-03T14:11:14Z

‎Software Design

Proj user18: /* Piezo buzzer */

2015-06-03T14:06:49Z

‎Piezo buzzer

Proj user18: /* Solenoid implementation */

2015-06-03T14:06:12Z

‎Solenoid implementation

Proj user18: /* LCD implementation */

2015-06-03T14:03:14Z

‎LCD implementation

Proj user18: /* LCD implementation */

2015-06-03T14:02:57Z

‎LCD implementation

Proj user18: /* LCD implementation */

2015-06-03T14:01:23Z

‎LCD implementation

Proj user18: /* LCD implementation */

2015-06-03T13:59:59Z

‎LCD implementation

Proj user18: /* Conclusion */

2015-06-03T05:11:04Z

‎Conclusion

@@ Line 258: / Line 258: @@
 Despite not being true SPI devices the shift registers work well for adding more I/O.
-The 74HCT595 has LVCMOS compatible inputs allowing it to interface with the 3.3V I/O of the LPC1758. It consists of an 8-bit shift register which is clocked by SCLK and fed data by MOSI. The shift register output signal (Q7') can be fed to the data input of additional shift registers. The shift register output feeds the input of an 8-bit output latch, which has a dedicated clock input. In this way you can shift any kind of data through the shift register chain without disturbing the actual output pins, and then only load data from the shift register to the output latch when necessary.
+The 74HCT595 has LVCMOS compatible inputs allowing it to interface with the 3.3V I/O of the LPC1758. It consists of an 8-bit shift register which is clocked by SCLK and fed data by MOSI. The shift register output signal (Q7') can be fed to the data input of additional shift registers. The shift register output feeds the input of an 8-bit output latch, which has a dedicated clock input. In this way you can shift
 The output of the final shift register in the chain can be fed back into MISO for diagnostic purposes; while the shift registers are output-only devices, connecting the output to MISO allows you to verify that the data transmitted through the chain was correctly received. This can help identify wiring errors or other problems early on and is a good diagnostic tool.

@@ Line 428: / Line 428: @@
 * main - User interface (main application).
 * myedlin - Line editor for user data input.
 * debug - Debugging and error handling functions.
 ====Main application operations and associated tasks====
-−
 The main application performs the following operations:
@@ Line 453: / Line 454: @@
 . Buzzer update task. High priority.
-This task generates a square wave to activate the piezo buzzer.
+This task generates a square wave to activate the piezo buzzer. To provide uninterrupted sound it is a high priority task.
 == Implementation ==

@@ Line 424: / Line 424: @@
 * myspi - SSP driver for SPI communication.
 * mysreg - Shift register abstraction.
 ====Software components====

@@ Line 391: / Line 391: @@
 of a given frequency, such as 440 Hz, creates a 440 Hz tone. At very low frequencies the buzzer can be used to make simple clicking
 sounds, such as to augment the keypad entry with clicks as each key is pressed. This kind of feedback makes it easier for the
-operator to tell if a key press was complete or not.
+user to tell if a key press was complete or not.
 A 0.1uF ceramic disc capacitor is placed across the buzzer terminals to filter out high frequency noise, and a 270-ohm resistor is

@@ Line 359: / Line 359: @@
 When the magnetic field collapses current is induced in the coil, causing a large voltage spike that can be damaging to any digital circuitry connected to it. To prevent this, a diode is placed in parallel with the solenoid coil with the cathode to the positive voltage supply (+12V). This provides a safe discharge path so that the voltage spike does not go into the digital circuitry.
 To control the the solenoid with a GPIO port, a ULN2803 is used, which is an eight channel Darlington transistor array. A Darlington transistor is a pair of transistors in series that can sink a large amount of current, and can interface to higher voltage signals such as 12V.
-The ULN2803 has built-in resistors to provide a TTL/CMOS compatible interface. Two important aspects to remember are the the Darlington transistors
+The ULN2803 has built-in resistors to provide a TTL/CMOS compatible interface.
-have an inverted output in relation to the input, such that an active-high input sinks current and an active-low input makes the output high
+The output is inverted with respect to the input, such that a high-level input sinks current on the output, and a low-level input makes the output go high impedance.
-impedance. Secondly, the high impedance output should have a pull-up or pull-down resistor to prevent the output from oscillating.
+Accordingly, the output must have a pull-up resistor to prevent any oscillation or noise when in the high impedance state.
-Note that the ULN2803 used has a built-in diode for this purpose, but during testing it was observed that the LCD display would flicker when the solenoid was turned off. Adding a large 1N4001 diode
+Each channel of the ULN2803 can handle 500mA. As there were multiple free channels available, three were used in parallel to provide up to 1.5A sinking ability, which is a little over twice of what the solenoid requires.
 [[File:S15_244_G29_IMG_0003.png|center]]

@@ Line 333: / Line 333: @@
 To be safe lengthy delays must be used which waste valuable time.
 It seemed much more worthwhile to have a hardware solution to enable polling, in order to keep the software responsive.
 ===LCD backlight control and contrast===

@@ Line 329: / Line 329: @@
 The HD44780 needs a certain amount of time to carry out actions, and will output a busy status flag on DB7 during a read cycle. As a output-only shift register was being used to drive the data bus it was not possible to read data. A hardware solution was to place a 10K resistor in series with the shift register, such that Q7 of the shift register could pull DB7 low or high. In this way the HD44780 could still drive DB7 which was connected to a GPIO, allowing the busy status to be read regardless of how the shift register was driving the pin.
-This was important as many people who use the HD44780 opt to disable the read function and use fixed delays instead of polling the busy flag. However the time it takes to complete operations is somewhat variable, and it varies further between different manufacturers. To be safe, lengthy delays are needed which waste valuable time. It seemed much more worthwhile to have a hardware solution to enable polling in this case, to keep the software responsive.
+This was important as many system-level uses of HD44780 opt to disable the read function entirely (grounding R/W#) and use fixed delays instead of polling the busy flag.
 The ULN2803 is used to drive the cathodes which vary in current but can be as high as 25mA, which is more than what a single GPIO can sink. Each channel of the ULN2803 can handle 500mA which is more than sufficient in this case.

@@ Line 327: / Line 327: @@
 [[File:S15_244_G29_IMG_0005.png|center]]
-The HD44780 needs a certain amount of time to carry out actions, and will output a busy status flag on DB7 during a read cycle. As a output-only shift register was being used to drive the data bus, a compromise was to insert a series 10K resistor such that Q7 of the shift register could pull DB7 low or high. In this the HD44780 could still drive DB7 to indicate the busy status without having a conflict with the shift
+The HD44780 needs a certain amount of time to carry out actions, and will output a busy status flag on DB7 during a read cycle. As a output-only shift register was being used to drive the data bus it was not possible to read data. A hardware solution was to place a 10K resistor in series with the shift register, such that Q7 of the shift register could pull DB7 low or high. In this way the HD44780 could still drive DB7 which was connected to a GPIO, allowing the busy status to be read regardless of how the shift register was driving the pin.
 This was important as many people who use the HD44780 opt to disable the read function and use fixed delays instead of polling the busy flag. However the time it takes to complete operations is somewhat variable, and it varies further between different manufacturers. To be safe, lengthy delays are needed which waste valuable time. It seemed much more worthwhile to have a hardware solution to enable polling in this case, to keep the software responsive.

@@ Line 323: / Line 323: @@
 * DB7-0 - Data bus (8-bits)
-To support devices with less GPIO such as 4-bit microcontrollers, the HD44780 has a 4-bit mode where only DB7-4 are used, and byte-sized data is transferred by writing four-bit nibbles twice in sequence. However the documentation is sparse on the exact timing in this mode, and it was unreliable when the control signals changed slowly. To compensate, one of the shift registers was allocated to provide all 8 bits of data, and the remaining PIO was used for the control signals.
+To support devices with less GPIO such as 4-bit microcontrollers, the HD44780 has a 4-bit mode where only DB7-4 are used, and byte-sized data is transferred by writing four-bit nibbles twice in sequence. However the documentation is sparse on the exact timing in this mode, and it was unreliable when the control signals changed slowly. To compensate, one of the shift registers was allocated to provide all 8 bits of data, and the remaining GPIO was used for the control signals. As the HD44780 is a slow device and the LPC1758 can change GPIO pins very quickly, it was critical to observe the timing characteristics in the datasheet.
 [[File:S15_244_G29_IMG_0005.png|center]]

← Older revision		Revision as of 05:11, 3 June 2015
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	A further exercise in security would be to use a wireless system (XBee) for the control box and security box to communicate.		A further exercise in security would be to use a wireless system (XBee) for the control box and security box to communicate.
		+
		+	=== Goals not completed ===
		+
		+	The RFID functionality was not finished. A functioning driver for the PN538 chip could not be developed.

	=== Other thoughts ===		=== Other thoughts ===