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Figure 9.19 The PIC microcontroller s parallel input/output pin consists of a data bit, a direction control bit, and a feedback bit, returning the state of the output pin.
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throughout this book to label individual pins. In some places you will see the convention PORT%.#, which uses the same values for % and # as R%#. The TRIS (tristate buffer enable) register is used to control the output capabilities of the I/O pin. When the register is loaded with a 1 (which is the power-up default), the pin is input-only (or in input mode) with the tristate buffer disabled and not driving the pin. When a 0 is loaded into a pin s TRIS bit, the tristate buffer is enabled (output mode), and the value that is in the data-out register is driven onto the pin. The TRIS register can be confusing in regard to where it is located in low-end and mid-range PIC microcontroller register maps. In low-end devices, the TRIS register can be written to using only the tris instruction. In mid-range and higher PIC microcontrollers, the TRIS register is often in a different bank from the port data register, which requires you to change the bank or use the tris instruction. These two methods are somewhat awkward to use, and both have their own quirks. The tris instruction has the format
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where # is A, B, or C. When this instruction executes, the contents of the w register is loaded into the TRIS register for the speci ed port. If the TRIS value is going to be updated in the application, then I recommend saving the value loaded into the TRIS register into a le register and updating that (and saving it again) before executing. The tris instruction code to do this would be
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movlw 0x0FF ^ (1 << Bit) andwf TRIS#SaveReg, w movf TRIS#SaveReg, w tris PORT# ; ; ; ; Find the Bit to Reset Update the Saved TRIS Value Use as the TRIS Value Write to the TRIS Register
The mid-range parts can execute the tris instruction, but it is not recommended by Microchip because support for it is not guaranteed for all future products. Personally, I would not recommend it for use with mid-range devices simply because not all the possible registers can be controlled with this instruction. In mid-range PIC microcontrollers, there can be ve I/O ports (PORTA through PORTE), the tris instruction can only access ports A, B, and C, which means that access to ports D and E is not possible. The recommended way of accessing mid-range PIC microcontroller TRIS registers is to change the RPO bit of the STATUS register and read or write the register directly using the code
bsf STATUS, RPO movlw NewTRISA movwf TRISA ^ 0x80 bcf STATUS, RPO
Note in this code that I XORed the TRISA register address with 0x80 to avoid any messages telling me to check the page that is being accessed. TRISA is a bank 1 register and
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has bit 7 set its address value is 0x85. XORing 0x85 with 0x80 will return a value of 0x05, which is a valid address within a mid-range PIC microcontroller bank. Bank addressing is explained in greater detail in Chap. 6. There is an important mnemonic to remember when you are setting or resetting a TRIS bit to put the register into input or output mode. Note that putting a port bit into output mode is accomplished by loading it with a 0, and putting it into input mode is accomplished by loading it with a 1. I always remember which is which by 0 output and 1 input. The digit approximates the rst letter of the corresponding word. Going back to Fig. 9.19, I want to point out that pin input is read from the I/O pin and not output of the data-out register. This is important to remember because there are occasions when this arrangement will cause problems. This issue is somewhat unique to low-end and mid-range PIC microcontrollers; other microcontrollers (including the PIC18) either have a separate input address or have a method of selecting which to read (the data-out register or the pin) based on whether or not the pin is in input or output mode. In Fig. 9.20 I show the organization of the PIC18 I/O pin with the LAT (or LATCH) register feedback showing the value stored in the PORT bit. The problem lies in the fact that many instructions read and write the register contents in ways that you do not expect. Probably the biggest culprits are the bcf/bsf instructions, which perform a register read, bit OR or AND, and then write the new value back to the register. There is the opportunity that a PORT bit will be in input mode with a speci c value set into it. If the pin is at a different value, then this value will be read and written back by the PIC microcontroller. The best way to avoid this problem is to always write the desired output values to the PORT register before changing the TRIS register. I wish I kept track of problems that people report on the PICList; I would bet that the number one or two problem encountered by people is with RA4 or PORTA.4 pin (the block diagram for which is shown in Fig. 9.21). This pin is an open-drain driver
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