UM001600-Z8X0599
3-1
U
SER
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S
M
ANUAL
C
HAPTER
3
C
LOCK
3.1 CLOCK
The Z8 MCU
derives its timing from on-board clock cir-
cuitry connected to pins XTAL1 and XTAL2. The clock cir-
cuitry consists of an oscillator, a divide-by-two shaping cir-
cuit, and a clock buffer. Figure 3-1 illustrates the clock
circuitry. The oscillator’s input is XTAL1 and its output is
XTAL2. The clock can be driven by a crystal, a ceramic
resonator, LC clock, RC, or an external clock source.
3.1.1 Frequency Control
In some cases, the Z8 has an EPROM/OTP option or a
Mask ROM option bit to bypass the divide-by-two flip flop
in Figure 3-1. This feature is used in conjunction with the
low EMI option. When low EMI is selected, the device out-
put drive and oscillator drive is reduced to approximately
25 percent of the standard drive and the divide-by-two flip
flop is bypassed such that the XTAL clock frequency is
equal to the internal system clock frequency. In this mode,
the maximum frequency of the XTAL clock is 4 MHz.
Please refer to specific product specification for availability
of options and output drive characteristics.
3.2 CLOCK CONTROL
In some cases, the Z8 offers software control of the internal
system clock via programming register bits. The bits are lo-
cated in the Stop-Mode Recovery Register in Expanded
Register File Bank F, Register 0BH. This register selects
the clock divide value and determines the mode of Stop-
Mode Recovery (Figure 3-2). Please refer to the specific
product specification for availability of this feature/register.
Figure 3-1. Z8 Clock Circuit
÷
2
OSC
XTAL2
Internal
Clock
Buffer
XTAL1
Figure 3-2. Stop-Mode Recovery Register
(Write-Only Except D7, Which is Read-Only)
D7 D6 D5 D4 D3 D2 D1 D0
SMR (F) OB
SCLK/TCLK Divide by 16
0 OFF **
1 ON
External Clock Divide Mode by 2
0 = SCLK/TCLK = XTAL/2*
1 = SCLK/TCLK = XTAL
* Default setting after RESET.
**Default setting after RESET and STOP-Mode Recovery.