W741C260 Datasheet, PDF (9/92 Pages) Winbond – 4-BIT MICROCONTROLLER

W741C260
Crystal
32 KHz or
400K to 4MHz
XIN1
or
Resistor
XOUT1
Crystal
32 KHz
XIN2
XOUT2
Figure 3. Oscillator Configuration
Dual-clock operation
This operation mode is selected by code option. In the dual-clock mode, the clock source of the LCD
frequency selector should be the sub-oscillator clock (32768 Hz) only. But in the single-clock mode,
the clock source of the LCD frequency selector will be Fm or Fm/32 (Fm: main oscillator clock).
In this dual-clock mode, the normal operation is performed by generating the system clock from the
main-oscillator clock (Fm). As required, the slow operation can be performed by generating the
system clock from the sub-oscillator clock (Fs). The exchange of the normal operation and the slow
operation is performed by resetting or setting the bit 0 of the system clock control register (SCR). If
the SCR.0 is reset to 0, the clock source of the system clock generator is the main-oscillator clock; if
the SCR.0 is set to 1, the clock source of the system clock generator is the sub-oscillator clock. In
dual-clock mode, the main-oscillator can stop oscillating when SCR.1 is set to 1. But in the single-
clock mode, the main-oscillator can not be stop from oscillating because the SCR would be disabled
in single-clock mode. Therefore, in sigle-clock mode, the clock source of the system clock generator
is the main-oscillator clock (FOSC = Fm).
When the SCR is set or reset, we must pay attention to the following:
1. X000B → X011B: Disable the main-oscillator (Fm) should not be done simultaneously with
changing the system clock source (FOSC) from Fm to Fs. The FOSC should be changed first from
Fm to Fs before the main-oscillator (Fm) is disabled. The correct seqence is:
X000B→X001B→X011B.
2. X011B → X000B: Enabling the main-oscillator (Fm) should not be done simultaneously with
changing the FOSC from Fs into Fm. The main-oscillator (Fm) should be enabled first before a
delay subroutine is called to allow the main-oscillator to oscillate stably. The FOSC can now be
changed from Fs into Fm. The correct sequence is therefore X011B→X001B→delay
subroutine→X000B. The suggested delay for Fm is 20 mS for 455 KHz ceramic resonator and 10
mS for 4 MHz crystal.
We must remember that the X010B state is inhibitive, because it will induce a system shutdown.
The organization of the dual-clock operation mode is shown below.
Publication Release Date: March 1998
-9-
Revision A3