ZiLOG Z8 Technical Manual

1K
+5V
100
KO
Figure 8-2. Power-Up Reset Circuit
When an external frequency source is used, it must
drive both XTAL1 and XTAL2 inputs. This differen-
tial drive requirement arises from the loading on
the oscillator output (XTAL2) without the reactive
feedback network of a crystal or resonator. A
typical clock interface circuit is shown in Figure
8-5.
The capacitors shown represent the maximum para-
sitic loading when using a 74LS04 driver. The
pull-up resistors can be eliminated by using a
74HC04 driver.
8.3 POWER-OOWN OPERATION
The Z8 has a power-down option which can be used
to maintain the contents of the register file with
a low-power
battery. The circuitry has its XTAL2
output replaced by 8 power supply input (V
MM
).
VM
M
powers
the general-purpose registers %04 -
%7F as well as a portion of the reset logic that
protects the register file. When V
MM
is main-
tained at 3 to 5 V, this power-down option pre-
serves the contents of the general-purpose regis-
ters whenever VCC is removed. During normal
operation, V
MM
provides +5 V along with VCC.
The following sequence is necessary to preserve
data:
• Power failure must be externally detected early
enough for a software routine to store the
required data that is not already in the regis-
ter file. An interrupt is typically used for
this purpose.
• RESET must be held Low after data is saved and
during
the removal of VCC. RESET is a write
protect input to the register file.
3047·062.3047·078,3047·079,3047·089
Reset and Clock
::~ ~ .. _o_s_c_ ... H .. __ +_2_ .. ~~L'ci'~~AL
Figure B-3. ZB Clock Circuit
Z8
Figure 8-4. Crystal/Ceramic Resonator Oscillator
CLOCK
IN
+5V +5V
1.5k 1.5k
74LS04 74LS04
><:>--..... - ....... ~~-+--e- XTAL2
I
CST RAY =
15 pF MAX
L...- _____ +_ XTAL1
I
CST RAY =
15 pF MAX
Figure 8-5. External Clock Interface
• RESET must be held Low during the power-up
sequence. Again,
RESET is a write protect
input to the register file.
As Vec powers down, on-board circuitry
associated with RESET automatically protects the
general-purpose registers. The circuit shown in
Figure 8-2 satisfies the power-up requirement of
holding RESET Low to protect the register file
data.
8-3

ZiLOG Z8 Specifications

General

Brand	ZiLOG
Model	Z8
Category	Desktop
Language	English