Motorola MC6805R Series - Page 45

(See Note 2) c=l
CL
External
~CIOCk
Input
NOTES:
I
6
XTAL
MCU
5
EXTAL
(Crystal Option,
See Note 1)
6 XTAL
MCU
5 EXTAL (Crystal Option,
See Note 1)
External Clock
6
XTAL
MCU
5
EXTAL
mc Option,
See Note 1)
Approximately 25% to 50% Accuracy
Typical tcyc= 1.25 p's
External Jumper
_'HH ___ 6 ... XTAL
R
(See Figure 7-5) 5 EXTAL
MCU
(RC Option,
See
Note 1)
No
Connection
Approximately 10% to 25% Accuracy
(Excludes Resistor Tolerance)
External Resistor
1. For the MC68705R3, MC68705U3, MC68705R5, and MC68705U5 MOR b7 = 0 for the crystal option and MOR b7 = 1 for the RC op-
tion. When the
TIMER input pin is in the VIHTP range (in the bootstrap EPROM programming mode), the crystal option is forced.
When the
TIMER input is at or below VCC, the clock generator option is determined by bit 7 of the Mask Option Register (CLK)'
2. The recommended CL value with a 4.0 MHz crystal is 27 pF maximum, including system distributed capacitance. There is an inter-
nal capacitance of approximately 25 pF on the XTAL pin. For crystal frequencies other than 4 MHz, the total capacitance on each
pin should
be scaled as the inverse of the frequency ratio. For example, with a 2 MHz crystal, use approximately 50 pF on EXTAL
and approximately
25 pF on XT AL. The exact value depends on the Motional-Arm parameters of the crystal used.
Figure 7-4. Clock Generator Options
When utilizing the on-board oscillator, the MCU should remain in a reset condition (reset pin
voltage below VIRES +) until the oscillator has stabilized at its operating frequency. Several factors
are involved in calculating the external reset capacitor required to satisfy this condition: the
oscillator start-up voltage, the oscillator stabilization time, the minimum VIRES +, and the reset
charging current specification.
Once VCC minimum is reached, the external RESET capacitor will begin to charge at a rate depen-
dent
on the capacitor value. The charging current is supplied from VCC through a large resistor, so
it appears almost like a constant current source until the reset voltage rises above VIRES +.
Therefore, the RESET pin will charge at approximately:
(VIRES + )eCext= IRESetRHL
Assuming the external capacitor is initially discharged.
7-3