MITSUBISHI MICROCOMPUTERS
M37754M8C-XXXGP, M37754M8C-XXXHP
M37754S4CGP, M37754S4CHP
SINGLE-CHIP 16-BIT CMOS MICROCOMPUTER
CLOCK GENERATING CIRCUIT
The clock generating circuit makes basic clocks, which activate the
central processing unit (CPU), bus interface unit (BIU) and internal
peripheral devices, of an oscillation circuit output. Figure 82 shows
the block diagram of the clock generating circuit.
XIN
XOUT
The clock source φ 1 to activate internal peripheral devices, the clock
source φ BIU to activate the bus interface unit and the clock source
φ CPU to activate the CPU are made of an clock input to the XIN pin.
When bit 6 (clock source select bit) of processor mode register 1 is
“0”, the clock which is obtained by dividing an input clock to the XIN
pin by 2 becomes the clock source φ 1. When bit 6 is “1”, the clock
which is an input clock to the XIN pin becomes the clock source φ 1
as it is. When bit 2 (clock source for peripheral devices select bit) is
“0”, the clock source φ 1 which is more divided by 2 becomes the
standard clock for peripheral devices. When bit 2 is “1”, the clock
source φ 1 becomes the standard clock for peripheral devices as it
is.
Rf
Rd
COUT
CIN
Fig. 80 Circuit example using a ceramic (or quartz crystal) resonator
The standard clock is more divided with the division circuit shown in
Figure 82 and the clocks having all kinds of frequencies are made.
Each internal peripheral device can select one of 4 clocks, Pf2, Pf16,
Pf64 and Pf512, and use it.
Pf2 means f(XIN), which is an oscillation circuit’s frequency, divided
by 2 when the clock source for peripheral devices select bit is “1”. It
means f(XIN) divided by 4 when that bit is “0”. In the case of φ 1 > 12.5
MHz, fix the bit to “0”.
XIN
XOUT
Open
Figure 80 shows a circuit example using a ceramic (or quartz crys-
tal) resonator. Use the manufactures’ recommended values for con-
stants such as capacitance which differs for each resonator.
Figure 81 shows a circuit example inputting clocks externally. When
inputting clocks externally, setting bit 1 (clock external input select
bit) of particular function select register 0 (in Figure 83) to “1” makes
operation of the clock oscillation circuit stop, that is, the XOUT output
stays at “H”, and the current consumption reduce.
External clock source
Vcc
Vss
Fig. 81 Circuit example inputting clocks externally
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RENESAS [ RENESAS TECHNOLOGY CORP ]
