92
CHAPTER 5 CLOCK GENERATOR
Preliminary User’s Manual U14581EJ3V0UM00
5.3 Clock Generator Control Registers
The following two types of registers are used to control the clock generator.
• Processor clock control register (PCC)
• Oscillator mode register (OSCM)
(1) Processor clock control register (PCC)
PCC sets the division ratio of the CPU clock.
PCC is set with a 1-bit or 8-bit memory manipulation instruction.
RESET input sets PCC to 04H.
Figure 5-2. Processor Clock Control Register (PCC) Format
Address: FFFBH After Reset: 04H R/W
Symbol
7
6
5
4
3
2
1
0
PCC
0
0
0
0
0
PCC2
PCC1
PCC0
PCC2
PCC1
PCC0
CPU Clock (f
CPU
) Selection
0
0
0
f
X
0
0
1
f
X
/2
0
1
0
f
X
/2
2
0
1
1
f
X
/2
3
1
0
0
f
X
/2
4
Other than above
Setting prohibited
Caution
Bits 3 to 7 must be set to 0.
Remark
f
X
: Main system clock oscillation frequency
The fastest instructions of the
µ
PD780852 Subseries are executed in two CPU clocks. Therefore, the relation
between the CPU clock (f
CPU
) and the minimum instruction execution time is as shown in Table 5-2.
Table 5-2. Relation between CPU Clock and Minimum Instruction Execution Time
CPU Clock (f
CPU
)
Minimum Instruction Execution Time: 2/f
CPU
f
CPU
= 8 MHz
f
CPU
= 8.38 MHz
f
X
0.25
µ
s
0.24
µ
s
f
X
/2
0.5
µ
s
0.48
µ
s
f
X
/2
2
1
µ
s
0.95
µ
s
f
X
/2
3
2
µ
s
1.91
µ
s
f
X
/2
4
4
µ
s
3.81
µ
s
Remark
f
X
: Main system clock oscillation frequency
Содержание mPD780851
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