7220 Datasheet, PDF (115/303 Pages) Renesas Technology Corp – 8-BIT SINGLE-CHIP MICROCOMPUTER

FUNCTIONAL DESCRIPTION
2.11 CRT display function
2.11.9 Clock for display
As a clock for display to be used for CRT display, it is possible to select one of the following 4 types.
q Main clock supplied from the XIN pin
q Main clock supplied from the XIN pin divided by 1.5
q Clock from the LC or RC supplied from the pins OSC1 and OSC2.
q Clock from the ceramic resonator or quartz-crystal oscillator supplied from the pins OSC1 and OSC2.
This clock for display can be selected by the CRT clock selection register (address 00ED16). When selecting
the main clock, set the oscillation frequency to 8 MHz.
CRT Clock Selection Register
b7 b6 b5 b4 b3 b2 b1 b0
000000
CRT clock selection register (CK) [Address 00ED16]
B
Name
Functions
0, 1 CRT clock ^{b1 b0}
Functions
selection bits 1 0 The clock for display is supplied by connecting RC
(CK0,CK1)
or LC across the pins OSC1 and OSC2.
After reset R W
0
RW
0 1 Since the main clock is used as the clock for _{CRT oscillation}
display, the oscillation frequency is limited. _frequency
Because of this, the character size in width _{= f(XIN)}
(horizontal) direction is also limited. In this
1 0 _{case, pins OSC1 and OSC2 are also used} CRT oscillation
as input ports P33 and P34 respectively.
frequency
= f(XIN)/1.5
1 1 The clock for display is supplied by connecting the
following across the pins OSC1 and OSC2.
• a ceramic resonator only for CRT display and a feedback resistor
• a quartz-crystal oscillator only for CRT display and a feedback
resistor (Note)
2 _{Fix these bits to “0.”}
to
7
0
RW
Note: It is necessary to connect other ceramic resonator or quartz-crystal oscillator across the pins XIN and XOUT.
Fig. 2.11.19 CRT clock selection register
2-88
7220 Group User’s Manual