Файл: Microcontroller Programming. Thi Micro Chip PIC (Julio Sanchez, 2007).pdf

The following code fragment lists a procedure to interface a 16F84 PIC with a 74HC165 parallel-to-serial shift register:

;============================================================

; constant definitions from wiring diagram

;============================================================

;============================================================

;74HC165 procedure to read parallel data and send

;============================================================

;OPERATION:

;1. Eight DIP switches are connected to the input

;ports of an 74HC165 IC. Its output line Hout,

;and its control lines CLK and load are connected

;to the PIC’s Port-B lines 0, 1, and 2

;respectively.

;2. Procedure sets a counter (bitCount) for 8

;iterations and clears a data holding register

;(dataReg).

;3. Port-B bits are read into w. Only the lsb of

;Port-B is relevant. Value is stored in a working

;register and the meaningful bit is rotated into

;the carry flag, then the carry flag bit is

;then shifted into the data register.

;4. The iteration counter is decremented. If this

;is the last iteration the routine ends. Otherwise

;the bitwise read-and-write operation is repeated.

The procedure in165 is in the program Serial6465 listed at the end of this chapter.

74HC164 Serial-to-Parallel Shift Register

The circuit in Figure 14-9 also uses a 74HC164 serial-to-parallel shift register for output to the eight LEDs. Figure 14-11 shows the pin-out of the 74HC164 IC.

Figure 14-11 74HC164 Pin Out

Serial input into the 164 is through the input A line (pin number 1). Parallel output is through the lines labeled Q0 to Q7. The reset/clear line (on pin 9) and the clock line (on pin 8) provide the control functions. The operations are as follows:

1.A local data storage register holds the 8-bit value that serves as data input. A local counter is initialized for 8 data bits.

2.The 164 shift register is cleared by pulsing the reset/clear line.

3.The first/next bit of the data operand is placed on the input line.

4.Bit is shifted-in by pulsing the 164 clock line.

5.Bit counter is decremented. If it goes to zero the routine ends.

6.Otherwise, the bits in the source operand are shifted and execution continues at step number 3.

The following code fragment lists a procedure to interface a 16F84 PIC with a 74HC164 serial-to-parallel shift register:

;=====================================================

; constant definitions from wiring diagram

;=====================================================

;============================================================

;74HC164 procedure to send serial data ;============================================================

;ON ENTRY:

;local variable dataReg holds 8-bit value to be

;transmitted through port labeled serialLN

;OPERATION:

;1. A local counter (bitCount) is initialized to

;8 bits

;2. Code assumes that the first bit is zero by

;setting the data line low. Then the high-order

;bit in the data register (dataReg) is tested.

;If set, the data line is changed to high.

;3. Bits are shifted in by pulsing the 74HC164

;clock line (CLK).

;4. Data bits are then shifted left and the bit

;counter is tested. If all 8 bits have been sent

;the procedure returns.

;Using this assumption is possible because the bit is not

;shifted in until the clock line is pulsed.

;=========================

;pulse clock line ;=========================

;Bits are shifted in by pulsing the 74HC164 CLK line

;=========================

return

It is important to note that serial communications that use shift register ICs are described as synchronous. Synchronous serial transmission requires that the sender and receiver use the same clock signal or that the sender provide signal or pulse so as to indicate to the receiver when to read the next data element from the line. In the circuits discussed in this section the shift/load, reset/clear, and clock lines provide this synchronous interface between the PIC and the shift register IC.

The program named Serial6465, in the book’s on line software, is a demonstration of PIC-to-shift register interfacing.

14.4 PIC Protocol-based Serial Programming

In the preceding sections we discussed circuits and developed software using PIC serial communications that did not conform to any particular protocol or standard. This style is adequate for stand-alone applications and circuits. On the other hand,

PIC-based circuits sometimes communicate with systems that conform to a specific communications standard, for example, with a PC through its RS-232-C serial port. In this case, the PIC software and hardware must conform with the protocol, at least to an operational minimum that ensures satisfactory interfacing with the protocol-based system.

In the context of protocol-based programming, two situations are possible: either the PIC in use supports the communications standard or protocol or it does not. In the case of the smaller PICs, such as the 16F84, the software emulates communications protocols since hardware provides no support. The more complex PICs, on the other hand, often contain hardware modules that provide a functionality equivalent to that required by the various standards. In this sense, mid-range and high-range PICs often include hardware support for one or more communication standards and conventions. For instance, the 16F87X PIC family includes an MSSP (Master Synchronous Serial Port) module and a USART (Universal Synchronous/asynchronous Receiver and Transmitter) module.

In the sections that follow we develop circuits and programs for cases in which the on-board PIC does not contain hardware support for the standard and for cases in which it does. Examples with PICs that do not provide hardware support for serial communications use the 16F84. Examples with PICs that provide hardware serial communications support use the 16F877, which contains an MSSP and a USART module. The 16F877 circuits and applications in the present chapter use the processor’s USART module. The 16F877 MSSP module is demonstrated in the chapter on EEPROM programming.

14.4.1 RS-232-C Communications on the 16F84

The UART (Universal Asynchronous Receiver/Transmitter) controller is a serial communications IC found in computers and other data communication devices. In the PC, the UART was originally National Semiconductor INS8250. With the introductions of the PC AT, IBM changed its serial IC to the NC16450, an improved 8250. Later PCs