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D Ctrl_Write Data Bus
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Figure 9.20 The PIC18 has a feedback register (LAT) that allows you to read back the contents of the PORT output latch.
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D Pin_Write
PORT Register
Pin_Read
Schmidt Trigger Input
Figure 9.21 The RA4 pin is an open-drain output instead of the typical totem-pole driver.
that can be used with other open-drain or open-collector drivers to create a dottedAND bus. When most people rst use RA4 (I m guilty of this as well), they forget or don t realize that the pin does not have the ability to drive a high voltage, and they can t understand why this pin seems to be broken. This pin cannot drive a positive voltage out unless it is pulled up (I normally use a 1 to 10 k resistor depending on the input capacitance of what is being driven). Another feature of some pins (including RA4) is that they have Schmit trigger inputs, which have a different threshold voltage for signals going low to high than those for going high to low, as shown in Fig. 9.22. The purpose of the Schmidt trigger is to provide hysteresis for an input signal and to try to eliminate some incorrect bouncing errors. This input causes RA4 to behave differently than other I/O pins in some circumstances. A feature that you should be aware of in mid-range parts is the availability of a controllable pull-up on the PORTB pins. This pull-up is controlled by the RPBU bit of the OPTION register and is enabled when this bit is reset and the bit itself is set for output. The port B pin block diagram is shown in Fig. 9.23. The weak pull-up is approximately
Typical Input Threshold (1/2 Vdd) Input Signal Low to High Threshold ( Rising Edge ) High to Low Threshold ( Falling Edge )
Figure 9.22 Schmit trigger inputs have threshold voltages unique to the direction of the edge and closer to the destination voltage than typical inputs, which switch at one-half Vdd in both directions.
BASIC OPERATING FEATURES
RPBU Vdd Q Weak Pull up Q
D Ctrl_Write
TRIS Register
Data Bus
D Pin_Write
PORT Register
Pin_Read
Figure 9.23 The weak pull-up built into PORTB pins eliminates the need for pull-up resistors.
50 k and can simplify button inputs, eliminating the need for an external pull-up resistor. Changing pin inputs can initiate interrupt requests to the processor. The function that is normally used is the RBO/INT pin, which can request an interrupt (if the INTE bit is set in the INTCON register). An interrupt request can be made on rising or falling edges as selected by the INTEDG bit of the OPTION register. If INTEDG is set, interrupts can be requested on the rising edge of signal into RBO/INT. INTEDG reset will cause an interrupt on a falling edge. I like to think of the RBO/INT interrupt request hardware as the block diagram shown in Fig. 9.24. While mid-range devices generally only have one interrupt request pin, PIC18 microcontrollers usually have several. Once the interrupt is acknowledged by the processor, the INTF bit of the INTCON register has to be reset to enable another interruption RBO/INT pin transition. I should note that the interrupt request goes through a Schmitt trigger buffer, whereas the standard pin feedback does not. This will affect the loading of the pin somewhat. The other type of interrupt that can be requested from the I/O pins is the port B change on interrupt. If the RBIE bit of the INTCON register is set, then any changes
INTEDG INTE Interrupt Request (Sets "INTF")
Schmit Trigger Input
Figure 9.24 The RB0/INT pin can be used to generate interrupt requests.
OPTION REGISTER
to the RB4 RB7 pins while they are in input mode will request an interrupt on port change and set the RBIF ag in the INTCON register. To clear this interrupt request, PORTB has to be rst read to set the current value that is followed by resetting the RBIF ag. The port change on interrupt is only available on RB4 RB7 when they are in input mode. Changing the state of any of these pins while they are in output mode will not cause a port change interrupt request. This interrupt is a bit tricky to use, but by remembering that PORTB should never be polled, you can avoid any situations where an unknown interrupt request resets before it can be acknowledged. To avoid this problem, when I use the port change interrupt feature, I do not make any other PORTB pins inputs, and I never use an instruction that could read the I/O port (such as movf, andwf, addwf, bcf, bsf, etc.).
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