Rainbow Electronics DS12C887 User Manual

Page 9

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DS12C887

9 of 19

REGISTER C

MSB

LSB

BIT 7

BIT 6

BIT 5

BIT 4

BIT 3

BIT 2

BIT 1

BIT 0

IRQF

PF

AF

UF

0

0

0

0

IRQF - The Interrupt Request Flag (IRQF) bit is set to a 1 when one or more of the following are true:
PF = PIE = 1
AF = AIE = 1
UF = UIE = 1

i.e., IRQF = (PF ● PIE) + (AF ● AIE) + (UF ● UIE)

Any time the IRQF bit is a 1, the

IRQ

pin is driven low. Flag bits PF, AF, and UF are cleared after

Register C is read by the program or when the RESET pin is low.

PF - The Periodic Interrupt Flag (PF) is a read-only bit which is set to a 1 when an edge is detected on the
selected tap of the divider chain. The RS3 through RS0 bits establish the periodic rate. PF is set to a 1
independent of the state of the PIE bit. When both PF and PIE are 1’s, the

IRQ

signal is active and will

set the IRQF bit. The PF bit is cleared by a software read of Register C or a RESET .

AF - A 1 in the Alarm Interrupt Flag (AF) bit indicates that the current time has matched the alarm time.
If the AIE bit is also a 1, the

IRQ

pin will go low and a 1 will appear in the IRQF bit. A RESET or a read

of Register C will clear AF.

UF - The Update Ended Interrupt Flag (UF) bit is set after each update cycle. When the UIE bit is set to
1, the 1 in UF causes the IRQF bit to be a 1, which will assert the

IRQ

pin. UF is cleared by reading

Register C or a RESET .

BIT 3 THROUGH BIT 0 - These are unused bits of the status Register C. These bits always read 0 and
cannot be written.

REGISTER D

MSB

LSB

BIT 7

BIT 6

BIT 5

BIT 4

BIT 3

BIT 2

BIT 1

BIT 0

VRT

0

0

0

0

0

0

0

VRT - The Valid RAM and Time (VRT) bit indicates the condition of the battery connected to the V

BAT

pin. This bit is not writeable and should always be a 1 when read. If a 0 is ever present, an exhausted
internal lithium energy source is indicated and both the contents of the RTC data and RAM data are
questionable. This bit is unaffected by RESET .

BIT 6 THROUGH BIT 0 - The remaining bits of Register D are not usable. They cannot be written and,
when read, they will always read 0.

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