NEC mPD780973 Series, Предварительное руководство пользователя

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CHAPTER 6 CLOCK GENERATOR
6.3 Clock Generator Control Register
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)
The PCC sets the division ratio of the CPU clock.
The PCC is set with a 1-bit or 8-bit memory manipulation instruction.
RESET input sets the PCC to 04H.
Figure 6-2. 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
) Select
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
PD780973 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 6-2.
Table 6-2. Relation between CPU Clock and Minimum Instruction Execution Time
CPU Clock (f
CPU
)
Minimum Instruction Execution Time: 2/f
CPU
f
X
= 8 MHz f
X
= 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
1.0
µ
s 0.95
µ
s
f
X
/2
3
2.0
µ
s 1.91
µ
s
f
X
/2
4
4.0
µ
s 3.81
µ
s
Remark f
X
: Main system clock oscillation frequency