mc68hc68t1 Freescale Semiconductor, Inc, mc68hc68t1 Datasheet - Page 24

• Manufacturer Part Number: mc68hc68t1
• Description: Mc68hc68t1 Real-time Clock Plus Ram With Serial Interface
• Manufacturer: Freescale Semiconductor, Inc

MC68HC68T1
24
1 TO 2 V p–p
NOTE: Refer to item 8.
1. The circuit works, but the standby current is well above
V BATT
0 V
2. When power is applied, the clock does not start up nor
3. The clock loses time, but the oscillator is tuned.
the spec. How can the standby current be reduced?
does it hold data in the control registers.
Make sure the POR circuit is connected and working.
Do not make constant accesses to the clock. When a read
or write cycle is started, the clock stops incrementing
time.
a. If using a 32.768 kHz crystal, include a series resistor
b. Connect the LINE pin to something other than V DD
c. Ensure that the Power–On–Reset (POR) has a time
d. Ensure that there is a diode from V DD to + 5 V of the
in the circuit per Figure 12 of the data sheet. A good
value to start with is 200 k
and XTAL in pins should look similar to Figure 23
when the correct value is selected. The sharp, clean
edges on the XTAL out pin reduces current on the
totem pole drivers internal to the device.
(e.g., V BATT , V SS , V SYS )
constant of at least 100 ms.
system,
Application Circuits.
APPROXIMATELY 30.52
Figure 23. XTAL Waveforms
TROUBLESHOOTING
in battery–backup applications. See
XTAL out
XTAL in
SEE NOTE
m
s
W
. The signals at XTAL out
10. Can the V SYS line be more than 0.5 V above V DD ?
11. The CLKOUT, CPUR, and PSE pins do not go inactive
4. When the part is power cycled, the clock loses all time and
5. Can a non–rechargeable lithium battery be used?
6. Able to read/write data to the RAM but not to the clock reg-
7. How is the oscillator tuned?
8. What is the accuracy of the oscillator?
9. Can the Line pin sense a dc failure?
data.
Check the battery installation and ensure that a diode is
in the circuit from V DD to + 5 V.
Yes, but the battery must have a large capacity. Careful
attention MUST be given if the end unit needs to be UL
approved. The circuit of Figure 22 is a good start.
isters, or vice versa.
There is a software problem. There is no internal differ-
ence from reading/writing to the RAM or clock locations.
The best way to tune the oscillator is to set the clock out
bits of the Clock Control Register (bits 0, 1, and 2) to
output the primary XTAL frequency (000). The frequency
can then be more accurately measured from the
CLKOUT pin. This prevents the measuring device from
loading the oscillator circuit, which may shift the fre-
quency.
The oscillator accuracy is dependent on the quality of the
crystal used. For every 1 ppm variance in crystal fre-
quency, the clock gains or loses 2.6 seconds per month.
25 ppm is a typical spec for a crystal, which translates to
"
Yes, the Line input is threshold triggered in a window from
one diode drop above and below V DD . If supply is
removed in the low cycle of a sine wave, the internal
network pulls the line pin to within the threshold in a few
milliseconds. In the absence of a dc voltage outside the
V DD
to within the window and triggers the interrupt.
No. There is an ESD protection network that causes a
supply problem with this application.
when V DD and V SYS are removed. The CLKOUT, CPUR,
and PSE are not active immediately when V DD and V SYS
is applied.
The problem is related to the power up procedure
(battery–backup mode or single–supply mode). See
these sections in the data sheet for more information.
65 seconds per month.
0.7 V window, the internal network pulls the signal
MOTOROLA