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Figure 9.3 You can design your own brown-out reset circuit using a comparator.
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POWER INPUT AND DECOUPLING
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of a reversed PIC microcontroller). This is one of the reasons why I like the 78(L)xx series of voltage regulators; they may be more expensive than some other parts, but they won t burn out when they experience overcurrent conditions or cause the parts they are driving to burn out. Many PIC microcontroller part numbers are identi ed as having nanoWatt technology. This is a reference to Microchip s advanced device manufacturing processes and the ability to consume as little as 50 nA (50 10 9 A) during sleep. This feature is important for battery-power applications, which can be put to sleep with no functions working other than the watchdog timer and wake on interrupt to ensure maximum battery life. Logitech s Harmony remotes take advantage of this feature by keeping the PIC microcontrollers built into them asleep except for the three cases of sending IR data, updating the LCD display, and being connected to a PC via USB.
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HIGH-VOLTAGE DEVICES
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As I go through the practical aspects of the PIC microcontroller, one aspect will seem annoying to do for every circuit, and that is creating a power-supply circuit with a voltage regulator with suf cient current rating to drive the circuit. While this is not terribly dif cult to do, it can take up valuable real estate and drive up the cost of your application. There are a number of PIC microcontrollers with built-in voltage regulators that allow the PIC microcontroller to be driven without any external regulators in applications where the power input is signi cantly higher than the nominal 2.0 to 6.0 V normally applied to a PIC microcontroller. These parts are given the letter value HV for high voltage and have a similar pinout as other, more standard PIC microcontroller part numbers, and the output of the voltage regulators is not made available outside the chips. To support the high-voltage PIC microcontrollers, there are a few tricks that you should be aware of, as well as some enhanced features that affect the operation of the part. To connect a PIC16HV540 (the rst high-voltage device to become available) to a battery, the circuit can be as simple as the one shown in Fig. 9.4. In this gure, I have
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PIC16HV540
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OSC 2
15V <= Power In <= 3.5V
PORT A T0CKI PORT B Vss
Figure 9.4 The power wiring of the PIC16HV540 is very similar to that of a PIC microcontroller with a regulated power supply.
BASIC OPERATING FEATURES
PIC16C54 Core
Voltage Regulator
PORT A PORT B Vss
15V <= Power In <= 3.5V
Figure 9.5 Assumed PIC16HV540 s internal power system block diagram.
shown the PIC16HV540 as being connected directly to a battery with only a 0.1- F decoupling capacitor. I don t show a switch because sleep can be used to turn off the device and put it into a low-current (no more than 14 A required) mode. Wakeup from sleep can be accomplished either by watchdog timer timeout, _MCLR, reset or a PORTB pin change. As I ve drawn the PIC16HV540, you may think that the device is similar to a PIC16C54 with a voltage regulator in front of it, as is shown in Fig. 9.5. This is not quite true because the I/O port pins may use the device s regulated voltage of the input voltage. The PORTA pins can provide up to the regulated voltage, whereas PORTB provides swings from ground to the input voltage. The actual device s block diagram looks like Fig. 9.6, and PORTA can be used to power 5/ 3 V TTL CMOS devices, whereas PORTB is well
PIC16C54 Core PORT A
Voltage Regulator
+ PORT B
Figure 9.6 diagram.
Actual PIC16HV540 internal power system block
CONFIGURATION FUSES
TABLE 9.1 BIT
PIC16HV540 OPTION2 REGISTER DEFINITION DESCRIPTION
7-6 5 4 3 2 1 0
Unused WPC When set, device will wake up on RB0 RB3 changing. SWE Software watchdog timer. If the WDT is not enabled in the con guration fuses, setting this bit will enable it in software. RL Regulated voltage select bit (set for 5 V; reset for 3 V). SL Sleep voltage level setting (if set, use RL voltage; when reset, use 3 V). BL Brown-out voltage select (when set 3.1 V for 5-V operation; when reset, 2.2 V for 3-V operation) BE Brown-out checking enabled when set.
suited for high-voltage I/O. PORTB s threshold voltage is similar to the PIC16C54 s (i.e., anything greater than 2.5 V is a 1) and can be used for buttons and LED I/O. In the PIC16HV540, the voltage regulator can work as either a 5- or 3-V regulator by setting or resetting, respectively, the RL bit of the OPTION2 register, which is in the OPTION/TRIS address space of the low-end PIC microcontroller processor. This register is an auxiliary con guration fuses register that can be modi ed within an application. The bits of the OPTION2 register are de ned as shown in Table 9.1. The PIC16HV540 is a low-end device, which means that there is a limited amount of space for hardware registers. To write to the OPTION2 register, which is not part of the standard memory map, the tris instruction is used as
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