Introduction to Microprocessors What makes a Pic Tick

Introduction to Microprocessors
What makes a Pic Tick
Course: ELN-232
(Start of Part 2)
This section will discuss what it takes to have a PIC microcontroller execute code. Originally when
microprocessors were developed, they required multiple voltages with tight tolerances. The clocks
were generated externally with multiple phases. The resetting of the processor was a complicated affair
with critical specifications of the power being active before releasing the reset line.
With the new PIC microcontrollers being built with CMOS fabrication, and clocks internally generated,
the designer’s job has gotten considerably easier. Let’s examine the power and clocking system for the
PIC microcontrollers.
Input Voltage Considerations
If you look at the PIC16F87XA datasheet (section 17.1, page 175) it shows that the voltage can range
from 4.0 to 5.5 volts. For the PIC16LF87XA it can go all the way down to 2.0 volts with limitations on
the oscillator frequency. What that says, is you could put three 1.5 volt dry cells in series and have 4.5
volts, enough to power a PIC.
What about the current requirements? Looking at the datasheet again (section 17.1, page 176) we see
that the 16F87XA has a maximum of 4 ma at 4MHz. You can even get the 16LF87XA down to 35
microamps, running at 32kHz! A battery would last quite a long time.
For line powered applications a simple wall plug power supply maybe all that is needed. But be careful
because the specifications on them may be misleading. They have a tendency to over-voltage until the
specified current is drawn before they achieve the published voltage. It is best to use a voltage
regulator following the wall transformer.
I have indicated 5 volts at 1 amp as the output specifications, but this is only true if the input wall plug
power supply can supply this current, and the LM7805 regulator has a large enough heat sink. I have
included a 47 uF (make sure it has a voltage rating greater than the input voltage) capacitor on the input
for regulator stabilization. The output can now be wired to the PIC microcontroller. Be sure to place
decoupling capacitors near the PIC power pins for getting rid of high frequency noise generated by the
PIC. There should be one or two 0.1uF ceramic capacitors and a 4.7uF tantalum capacitor to supply
the ceramic caps when they need current. The 0.1uF is for high frequency and the 4.7uf is for low
frequency noise removal on the 5 volt power rail.
If you are only powering the PIC and low current devices, you might be able to use a small TO-92

device, like the LM78L05, which can put out 100ma of current. Just make sure you include in the
current calculation all loading on the pins of the PIC. Even though the PIC might only draw 5ma with
no pins connected, you can supply up to 25ma per pin. That can add up fast when powering LEDs and
driving low value resistors (never less than 200 ohms) into the bases of transistors.
If you are worried about efficiency, then the LM7805 is not a good choice. If you are drawing 500ma
and the input voltage is 25volts, then the drop across the LM7805 is 25 – 5, or 20 volts. The output
supplied wattage is 5 volts * 0.5 amps = 2.5 watts, where the dissipation in the regulator is 20 volts *
0.5 amps = 10 watts. That says the efficiency is 2.5 / (10 + 2.5) = 0.2 or 20%. Not very good
considering that the 10 watts is just going up in heat and would require a large heat sink. So keep the
input voltage as low as possible, making sure it is above the minimum required 7.0 volts. At 7.0 volts
the dissipation in the regulator would be 7 – 5 * 0.5 = 1 watt.
PIC Reset Conditions
Brown-Out-Reset (BOR)
There is protection in the PIC for when the input voltage falls below the point at which the
microcontroller may not function correctly. This is the brown-out reset (BOR). If the Vdd input
voltage falls below 4 volts, and if the BOR is enabled (BOREN = 1), then the processor will be reset.
When the voltage raises above 4 volts the BOR still stay enabled for 72 ms to ensure proper startup
operation.
Power On Reset (POR)
Another reset that is in the PIC is the Power on Reset (POR) circuit. This is the main reset mechanism
when the input voltage is being established. To make sure the POR functions properly the incoming
Vdd voltage must raise faster than 0.05 V/ms. This is from the section 17.1 on page 175. So the power
supply must be designed to accommodate this specification. When the input voltage has risen above
1.2 to 1.7 volts (internally set), the POR generates a reset pulse. An internal timer makes sure that the
controller stays in the reset condition for 72 ms, but it can be disabled with -PWRT set to 1 in the
PCON register.
Master Clear (-MCLR)
Also involved in the powering up sequence is the negative active Master Clear (–MCLR) input line. To
force an external reset, this line is pulled low, like with a pushbutton connected to ground. The line
should be pulled up with a resistor and a capacitor to ground, for noise. Microchip recommends that a
resistor greater than 1k ohm be put in line with the –MCLR pin. See figure 14-5 on page 148. I chose
a 2.2k resistor in the Lab. The pull up resistor I use is a 4.7k with a 0.01uF ceramic capacitor to
ground.
Oscillator Start-up Timer (OST)
The Oscillator Start-up Timer is active for 1024 oscillator clocks (OSC1 input), after PWRT concludes
and helps to ensure the oscillator has stabilized. It is only activated for XT, LP and HS modes.
Power Control Register (PCON)
Section 2.2.2.8 on page 29 actually describes the individual bits, but section 14.10 on page 149
describes the Power Control and Status Register, with the following page 150 showing the power on
reset state of all the registers.

Oscillator Circuit
Enough about reset circuits, lets get to where the microcontroller actually does something. The
oscillator is the crank that sequences all the states in the PIC. The controls for the oscillator clock
circuit can range from simple, a resistor and capacitor, to the complex with a crystal and stabilizing
capacitors. You have to tell the microcontroller what type of circuit to expect by programming
configuration bits to set modes. The following modes are available:
LP Low-Power Crystal
XT Crystal/Resonator
HS High-Speed Crystal/Resonator
RC Resistor/capacitor
All the Oscillator types are described in Section 1.2 on page 145. It is basically cost verses accuracy.
The RC network is cheap but it can drift and the initial accuracy is poor. To be able to time things well
you have to use a crystal which can add cost. The crystal will allow you to drive the PIC to higher
frequencies than the RC network. In the lab we will be using an RC network.
On newer PICs ( like the PIC16F628A and PIC16F690 ) there are additional oscillator types:
INTOSC Internal Precision 4 MHz Oscillator
EC External Clock In
Instruction Sequencing
The oscillator sends a clock signal to the Timing Generator. The Timing Generator sequences the
Instruction Decode and Control. This is the heart of the microcontroller.
Figure 1: PIC Instruction Execution Timing Diagram
As an instruction goes through execution, the next instruction is already being retrieved from the
Program Memory. This overlap helps speed up execution, eliminating the wait for the instruction to be
fetched from Program Memory. The key thing to remember from this timing diagram is it takes 4
oscillator cycles to execute one instruction. We will use this timing information quite a lot in the
future.
Most instructions in the PIC execute in one instruction cycle, which is 4 clock cycles, (Refer to the
datasheet’s table 15-2 on page 160) like the ADDWF, ANDWF, INCF and DECF. The ones that don’t
are ones that change the program counter from incrementing to the next instruction. These would be
like CALL, RETURN and GOTO.
(End of Part 2)