SC2200 NSC [National Semiconductor], SC2200 Datasheet - Page 119

• Manufacturer Part Number: SC2200
• Description: Thin Client On a Chip
• Manufacturer: NSC [National Semiconductor]

Revision 3.0
SuperI/O Module
4.5.2.9
The RTC has a single Interrupt Request line which handles
the following three interrupt conditions:
• Periodic interrupt.
• Alarm interrupt.
• Update end interrupt.
The interrupts are generated if the respective enable bits in
the CRB register are set prior to an interrupt event occur-
rence. Reading the CRC register clears all interrupt flags.
Thus, when multiple interrupts are enabled, the interrupt
service routine should first read and store the CRC regis-
ter, and then deal with all pending interrupts by referring to
this stored status.
If an interrupt is not serviced before a second occurrence
of the same interrupt condition, the second interrupt event
is lost. Figure 4-12 illustrates the interrupt timing in the
RTC.
Bit 7
of CRA
Bit 4
of CRC
Bit 6
of CRC
Bit 5
of CRC
Flags (and IRQ) are reset at the conclusion of CRC read or by
reset.
A = Update In Progress bit high before update occurs = 244 s
B = Periodic interrupt to update = Period (periodic int) / 2 +
C = Update to Alarm Interrupt = 30.5 s
P = Period is programmed by RS[3:0] of CRA
244 s
Figure 4-12. Interrupt/Status Timing
Interrupt Handling
B
244
P/2
30.5
(Continued)
s
P
s
A
C
P/2
119
4.5.2.10 Battery-Backed RAMs and Registers
The RTC has two battery-backed RAMs and 17 registers,
used by the logical units themselves. Battery-backup power
enables information retention during system power down.
The RAMs are:
• Standard RAM
• Extended RAM
The memory maps and register content of the RAMs is
provided in Section 4.5.4 "RTC General-Purpose RAM
Map" on page 124.
The first 14 bytes and 3 programmable bytes of the Stan-
dard RAM are overlaid by time, alarm data and control reg-
isters. The remaining 111 bytes are general-purpose
memory.
Registers with reserved bits should be written using the
read-modify-write method.
All register locations within the device are accessed by the
RTC Index and Data registers (at base address and base
address+1). The Index register points to the register loca-
tion being accessed, and the Data register contains the
data to be transferred to or from the location. An additional
128 bytes of battery-backed RAM (also called Extended
RAM) may be accessed via a second pair of Index and
Data registers.
Access to the two RAMs may be locked. For details see
Table 4-7 on page 108.
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