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PIC16F84A Datasheet(PDF) 25 Page - Microchip Technology |
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PIC16F84A Datasheet(HTML) 25 Page - Microchip Technology |
25 / 124 page ![]() PIC16F8X © 1998 Microchip Technology Inc. DS30430C-page 25 5.3 I/O Programming Considerations 5.3.1 BI-DIRECTIONAL I/O PORTS Any instruction which writes, operates internally as a read followed by a write operation. The BCF and BSF instructions, for example, read the register into the CPU, execute the bit operation and write the result back to the register. Caution must be used when these instructions are applied to a port with both inputs and outputs defined. For example, a BSF operation on bit5 of PORTB will cause all eight bits of PORTB to be read into the CPU. Then the BSF operation takes place on bit5 and PORTB is written to the output latches. If another bit of PORTB is used as a bi-directional I/O pin (i.e., bit0) and it is defined as an input at this time, the input signal present on the pin itself would be read into the CPU and rewritten to the data latch of this particular pin, overwriting the previous content. As long as the pin stays in the input mode, no problem occurs. However, if bit0 is switched into output mode later on, the content of the data latch is unknown. Reading the port register, reads the values of the port pins. Writing to the port register writes the value to the port latch. When using read-modify-write instructions (i.e., BCF, BSF, etc.) on a port, the value of the port pins is read, the desired operation is done to this value, and this value is then written to the port latch. A pin actively outputting a Low or High should not be driven from external devices at the same time in order to change the level on this pin (“wired-or”, “wired-and”). The resulting high output current may damage the chip. 5.3.2 SUCCESSIVE OPERATIONS ON I/O PORTS The actual write to an I/O port happens at the end of an instruction cycle, whereas for reading, the data must be valid at the beginning of the instruction cycle (Figure 5-5). Therefore, care must be exercised if a write followed by a read operation is carried out on the same I/O port. The sequence of instructions should be such that the pin voltage stabilizes (load dependent) before the next instruction which causes that file to be read into the CPU is executed. Otherwise, the previous state of that pin may be read into the CPU rather than the new state. When in doubt, it is better to separate these instructions with a NOP or another instruction not accessing this I/O port. Example 5-1 shows the effect of two sequential read-modify-write instructions (e.g., BCF, BSF, etc.) on an I/O port. EXAMPLE 5-1: READ-MODIFY-WRITE INSTRUCTIONS ON AN I/O PORT ;Initial PORT settings: PORTB<7:4> Inputs ; PORTB<3:0> Outputs ;PORTB<7:6> have external pull-ups and are ;not connected to other circuitry ; ; PORT latch PORT pins ; ---------- --------- BCF PORTB, 7 ; 01pp ppp 11pp ppp BCF PORTB, 6 ; 10pp ppp 11pp ppp BSF STATUS, RP0 ; BCF TRISB, 7 ; 10pp ppp 11pp ppp BCF TRISB, 6 ; 10pp ppp 10pp ppp ; ;Note that the user may have expected the ;pin values to be 00pp ppp. The 2nd BCF ;caused RB7 to be latched as the pin value ;(high). FIGURE 5-5: SUCCESSIVE I/O OPERATION PC PC + 1 PC + 2 PC + 3 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Instruction fetched RB7:RB0 MOVWF PORTB write to PORTB NOP Port pin sampled here NOP MOVF PORTB,W Instruction executed MOVWF PORTB write to PORTB NOP MOVF PORTB,W PC TPD Note: This example shows a write to PORTB followed by a read from PORTB. Note that: data setup time = (0.25TCY - TPD) where TCY = instruction cycle TPD = propagation delay Therefore, at higher clock frequencies, a write followed by a read may be problematic. |
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