Functional Description
(Continued)
Both of the flags and the interrupt output are reset by the
trailing edge of the read strobe The flag information is held
latched during a control register read guaranteeing that sta-
ble status information will always be read out by the proces-
sor
Interrupt timeout is detected and stored internally if it occurs
during a read of the control register the interrupt output will
then go low only after the read has been completed
A clock setting pulse occurring during a control register read
will
not
affect the data-changed flag since time data read
out before or after the control read will not be affected by
the time change
METHODS OF DEVICE OPERATION
Test Mode
National Semiconductor uses test mode for functionally
testing the MM58274C after fabrication and again after
packaging Test mode can also be used to set up the oscil-
lator frequency when the part is first commissioned
1) Disable interrupt on the processor to allow oscillator set-
ting Write 15
10
into the control register
The clock and inter-
rupt start stop bits are set to 1 ensuring that the clock and
interrupt timers are both halted Test mode and the interrupt
register are selected
2) Write 0 to the interrupt register
Ensure that there are no
interrupts programmed and that the oscillator will be gated
onto the interrupt output
3) Set oscillator frequency
All timing has been halted and
the oscillator is buffered out onto the interrupt line
4) Write 5 to the control register
The clock is now out of test
mode but is still halted The clock setting register is now
selected by the interrupt select bit
5) Write 0001 to all registers This ensures starting with a
valid BCD value in each register
6) Set 12 24 Hours Mode
Write to the clock setting register
to select the hours counting mode required
7) Load Real-Time Registers
All time registers (including
Leap Years and AM PM bit) may now be loaded in any
order Note that when writing to the clock setting register to
set up Leap Years and AM PM the Hours Mode bit must
not be altered from the value programmed in step 5
8) Write 0 to the control register
This operation finishes the
clock initialization by starting the time The final control reg-
ister write should be synchronized with an external time
source
In general timekeeping should be halted before the time
data is altered in the clock The data can however be al-
tered at any time if so desired Such may be the case if the
user wishes to keep the clock corrected without having to
stop and restart it i e winter summer time changing can be
accomplished without halting the clock This can be done in
software by sensing the state of the data-changed flag and
only altering time data just after the time has rolled over
(data-changed flag set)
Figure 4
shows the internal clock connections when the de-
vice is written into test mode The 32 768 kHz oscillator is
gated onto the interrupt output to provide a buffered output
for initial frequency setting This signal is driven from a
TRI-STATE output buffer enabling easy oscillator setting in
systems where interrupt is not normally used and there is no
external resistor on the pin
If an interrupt is programmed the 32 768 kHz output is
switched off to allow high speed testing of the interrupt tim-
er The interrupt output will then function as normal
The clock start stop bit can be used to control the fast
clocking of the time registers as shown in
Figure 4
Initialization
When it is first installed and power is applied the device will
need to be properly initialized The following operation steps
are recommended when the device is set up (all numbers
are decimal)
TL F 11219 – 7
FIGURE 4 Test Mode Organization
9
NSC [ NATIONAL SEMICONDUCTOR ]
