14.2 Parallel Data Transmission

In PIC-based systems, parallel communications often refer to the general principle rather than to the specific Centronics implementation. For example, wiring an 8-line toggle switch to the eight pins of the 16F85 Port-B line provides parallel communications between the switch and the PIC.

PIC circuits that use parallel data transfers offer many advantages. In the first place, parallel transmission is fast and the software is simple to develop. The hardware implementation is straightforward and does not require many additional components. Examples are connecting a multiple toggle switch to each of the lines of a PIC input port, or each of the pins of a seven-segment LED to the various pins of a PIC output port. The disadvantages of parallel systems are the distance limitations and the cost in system resources. Furthermore, parallel data transfers do not work well for data transmission over long distances. Many of the circuits and programs covered in previous chapters use parallel data transmission techniques. Since

PIC-based systems rarely communicate with parallel printers or use the Centronics standard for data transfer, no further discussion of the Centronics standard is justifiable in this context.

14.2.1 PIC Parallel Slave Port (PSP)

Some PICs are equipped with an 8-bit Parallel Slave Port module (PSP). At present, the PSP is multiplexed onto Port D and is found in PICs of the mid-range family, such as the 16F877. The PSP is also called the microprocessor port.

The PSP module provides an interface mechanism with one or more microprocessors. The parallel slave port has an operating speed of 200 ns with a clock rate of 20 MHz, as well as several on-chip peripheral functions for implementing real world interfaces.

In PICs equipped with the PSP, the parallel slave port functions are assigned to Port D, with some Port E bits providing control signals. To initialize PSP mode, data direction bits in the TRISE register that correspond to RD, WR, and CS (TRISE<2:0>) are configured as inputs and the control bit PSPMODE (TRISE) is set. When the PSP mode is active, Port D is asynchronously readable and writable through the chip Select (RE2/CS), Read (RE0/RD), and Write (RE1/WR) control inputs.

At this time, not many general-purpose applications for the PSP port have been documented, outside of its use as a multi-microprocessor interface. For this reason we have excluded PSP programming from this context.

14.3 PIC “Free-style” Serial Programming

This section is about PIC serial programming and circuit design that does not follow any specific communications protocol. In this sense, we have used the expression “free-style” as opposed to circuits and programs constrained by the requirements of a standard or convention. Many self-contained PIC circuits that do not interface with standardized components can benefit from not having to follow any specific standard. Later in this chapter, and in other chapters in the book, we present examples of PIC circuits and programs that follow established communications protocols. The titles of

the corresponding sections refer to the specific standards or protocols; for example, the section titled PIC RS-232-C Serial Programming found in this chapter.

The advantages of so-called “free-style” circuit design and programming are greater in ease in development and the use of fewer hardware components. When designer and programmer are not constrained by the specifications of a standard, the circuit can be implemented with a minimal number of hardware components. By the same token, software is simpler and easier to develop.

The following examples of free-style communications systems are presented in the sections that follow:

1.A PIC to PIC communications circuit and program. Two programs are required: one for the receiver PIC and one for the sender.

2.Serial-to-parallel and parallel-to-serial circuit and program. Circuit uses 74HC164 and

74HC165 ICs.

14.3.1 PIC-to-PIC Serial Communications

Perhaps the most obvious and straightforward mode of PIC serial communications is one that takes place between two PICs. In this case, one PIC acts as a sender, or master, and the other one as a receiver or slave, although it is also possible for sender and receiver to exchange roles. Consider a circuit in which one PIC polls the state of a bank of switches and then sends the result serially to a second PIC that controls a bank of LEDs to be lighted according to the switch settings. The reason for this circuit is that some PICs may not have a sufficient number of ports to monitor eight switches and control eight LEDs.

PIC-to-PIC Serial Communications Circuits

Actually, the system required for one PIC reading data and serially sending the result to another PIC that outputs the data can be visualized as two separate circuits. One circuit is used to read the state of the eight DIP switches and to send the data serially to another PIC circuit that displays the results. Figure 14-7 shows the two PIC-based circuits.

Structurally, the circuits in Figure 14-7 are quite similar to ones described previously in this book. The bottom circuit contains eight DIP switches wired to ports RB0 to RB7. A pushbutton switch is wired to port RA2 and a LED to port RA3. The serial output is through port RA1. The circuit at the top of Figure 14-7 has eight LEDs wired to ports RB0 to RB7. There is a pushbutton on port RA2 and a LED on port RA3. Input into the circuit is through port RA0. In the remainder of this description we refer to the bottom circuit as the sender circuit and PIC and the one on the top as the receiver circuit and PIC.

The pushbuttons are necessary so that sender and receiver are synchronized. In operation, the receiver circuit is first activated by pressing the switch labeled “receive ready.” The LED on the top circuit lights to indicate the ready state. The sender circuit has a LED labeled “ready” that indicates its state. The user presses the switch labeled “send ready” in the sender circuit. At that time, the program in the sender reads the state of the DIP switches and sends the data out, one bit at a time, through the line labeled “serial out” in the diagram. The receiver reads the eight bits in its “serial in” line and lights the LEDs accordingly.

The sender program, named SerialSnd, performs the following initialization operations:

1.Line RA2 is initialized for input since the pushbutton switch is located on this line. Lines RB0 to RB7 are also input, since they are connected to the DIP switch array.

2.The prescaler is assigned to the Watchdog Timer so that channel TMR0 runs at full processor speed.

3.Interrupts are disabled.

Initialization code is as follows:

; Port-A, bit 2 is input. All others are output

;Prepare to set prescaler clrf tmr0 clrwdt

;Setup OPTION register for full timer speed

;|__________________________ RBPU pullups

Once initialized, the program performs the following functions:

1.The SEND READY LED is turned on.

2.Code monitors the SEND pushbutton switch.

3.Once the switch is pressed, the program turns off the SEND READY LED.

4.The state of the DIP switches is obtained by reading RB0 to RB7.

5.The byte from Port-B is sent through the serial line.

The following code fragment shows the procedure to send serial data.

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

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

;ON ENTRY:

;local variable dataReg holds 8-bit value to be

;transmitted through port labeled serialLN

;OPERATION:

;1. The timer at register TMR0 is set to run at

;maximum clock speed, that is, 256 clock beats.

;The timer overflow flag in the INTCON register

;is set when the timer cycles from 0xff to 0x00.

;2. Each bit (start, data, and stop bits) is sent

;at a rate of 256 timer beats. That is, each bit is

;held high or low for one full timer cycle (256

;clock beats).

;3. The procedure tests the timer overflow flag

;(tmrOVF) to determine when the timer cycle has

;ended, that is when 256 clock beats have passed.

;

sendData:

movwf bitCount

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

;send START bit ;=======================

;Set line low then hold for 256 timer clock beats.

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

;send 8 DATA bits ;========================

;Eight data bits are sent through the serial line

;starting with the high-order bit. The data byte is

;stored in the register named dataReg. The bits are

;rotated left to the carry flag. Code assumes the bit

;is zero and sets the serial line low. Then the carry

;flag is tested. If the carry is set the serial line

;is changed to high. The line is kept low or high for

;256 timer beats.

send8:

;Code can assume the bit is a zero and set the line

;low since, if low is the wrong state, it will only

;remain for two timer beats. The receiver will not

;check the line for data until 128 timer beats have

;elapsed, so the error will be harmless. In any case,

;there is no assurance that the previous line state is

;the correct one, so leaving the line in its previous

;state could also be wrong.

;At this point timer has cycled.

;Test for end of byte, if not, send next bit

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

;hold MARKING state ;=========================

;All 8 data bits have been sent. The serial line must

;now be held high (MARKING) for one clock cycle

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

; end of transmission

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

return

The code comments explain the routine’s operation.

The receiving program, named SerialRcv, runs in the receiver PIC. In this case, the serial line is RA0. Input from the sender program is received through this line. The program performs the following initialization operations:

1.Lines RA0 and RA2 are initialized for input since the pushbutton switch is located on RA2 and RA0 is the serial input line. Lines RB0 to RB7 are output since they are wired to the eight LEDs.

2.The prescaler is assigned to the Watchdog Timer so that channel TMR0 runs at full processor speed.

3.Interrupts are disabled.

Once initialized, code performs the following functions:

1.The SEND READY LED is turned on.

2.Code monitors the RECEIVE READY pushbutton switch.

3.Once the switch is pressed, the program turns on the RECEIVE READY LED.

4.Code then monitors the serial line for the first low that indicates the leading edge of the start bit.

5.Once the start bit is detected, code waits for 128 clock cycles to locate the center of the start bit. This synchronizes the receiver with the sender and accommodates small timing errors.

6.The eight data bits are then received and stored.

7.After waiting for the stop bit, code turns off the RECEIVE READY LED and sets the eight LEDs according to the data received through the serial line.

The following code fragment is the procedure rcvData from the SerialRcv program:

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

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

;ON ENTRY:

;local variable dataReg is used to store 8-bit value

;received through port (labeled serialLN)

;OPERATION:

;1. The timer at register TMR0 is set to run at

;maximum clock speed, that is, 256 clock beats.

;The timer overflow flag in the INTCON register

;is set when the timer cycles from 0xff to 0x00.

;2. When the START signal is received, the code

;waits for 128 timer beats so as to read data in

;the middle of the send period.

;3. Each bit (start, data, and stop bits) is read

;at intervals of 256 timer beats.

;4. The procedure tests the timer overflow flag

;(tmrOVF) to determine when the timer cycle has

; ended, that is when 256 clock beats have passed.

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

rcvData:

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

; wait for START bit

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

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

;offset 128 clock beats ;=========================

;At this point the receiver has found the falling

;edge of the start bit. It must now wait 128 timer

;beats to synchronize in the middle of the sender’s

;data rate, as follows:

Neither the SerialRcv nor the SerialSnd programs contain any handshake signal. The programs rely on the user turning-on the receiver before the send function is activated. If this is not the case, the programs fail to communicate. But looking at the circuit diagram in Figure 14-7, we notice that there are available ports in both receiver and sender circuits. The circuit designer could interconnect two ports, one in the receiver and one in the sender, so as to provide a handshake signal.

For example, lines RA4 in both circuits can be interconnected. Then Port-A, line 4, in the sender circuit is defined as input and the same line as output in the receiver. The receiver could then set the handshake line high to indicate that it is ready to receive. The sender monitors this same port and does not start the transmission of each character until it reads that the handshake line is high. In this manner, the receiver can suspend transmission at any time and prevent data from being lost. At the same time, the “receiver ready” and “send ready” LEDs can be eliminated.

14.3.2 Program Using Shift Register ICs

The problem of handling multiple input and output lines, which was resolved in the previous example by using two PICs, can also be tackled by means of special-purpose integrated circuits. The term shift register refers to the fact that register input and output are connected in a way that data is shifted-down a set of flip-flops when the circuits are activated. Many variations of shift registers ICs are available, the most popular ones being serial-in to serial-out, parallel-in to parallel-out, serial-in to parallel-out, and parallel-in to serial-out. In shift register terminology the in and out terms refer to the function in the registers themselves, and are not related to the functions that these elements perform in a particular circuit. Figure 14-9 shows an input/output circuit using shift registers.

The circuit in Figure 14-9 shows the use of a parallel-to-serial IC (74HC165) that reads the state of eight input switches, and a serial-to-parallel IC (74HC164) that outputs data to eight LEDs. Without the shift register ICs, the circuit would require sixteen ports, more than those available in the 16F84. Using the shift registers, only six PIC ports are required, leaving eight ports available on the PIC. The demonstration program for the circuit in Figure 14-9 is named Serial6465.

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