Introduction to the HCS12 Microcontroller - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 13

D Q

CLK

Set Reset

Figure 1.3 ■ Block diagram of a D flip-flop with a set and reset

• The Q signals of all the flip-flops of the program counter determine the address of
the next instruction to be fetched.
• The set and reset inputs are active low (low voltage means logic 1) and ca nnot be
low at the same time. When set is low, the Q signal is forced to 1. When reset is
low, the Q signal is forced to 0.
As described in Section 1.3.1, a microprocessor or microcontroller has instructions to
change the program flow. The design of the program counter circuit must take this into account.
Figure 1.4 shows the block diagram of a program counter of an 8-bit microcontroller that allows
the program counter to be
• Forced to 0
• Incremented by 1
• Incremented by a field in the IR
• Loaded with a jump target

1

Branch
offset

01
MUX1

Branch

16

Adder

Q s

16

CLK Clk

16

D s

Jump
target

16 Power

on

Reset

Jump

Figure 1.4 ■ A simplified block diagram of the program counter (PC) of an 8-bit microcontroller