7

Detailed Description

Oscillator

The ICM7170 has an onboard CMOS Pierce oscillator with an

internally regulated voltage supply for maximum accuracy,

stability, and low power consumption. It operates at any of

four popular crystal frequencies: 32.768kHz, 1.046576MHz,

2.097152MHz, and 4.194304MHz (Note 1). The crystal

should be designed for the parallel resonant mode of

oscillation. In addition to the crystal, 2 or 3 load capacitors are

required, depending on the circuit topology used.

The oscillator output is divided down to 4000Hz by one of

four divider ratios, determined by the two frequency

selection bits in the Command Register (D0 and D1 at

address 11H). This 4000Hz is then divided down to 100Hz,

which is used as the clock for the counters.

Time and calendar information is provided by 8 consecutive,

programmable counters: 100ths of, seconds, minutes,

hours, day of week, date, month, and year. The data is in

binary format with 8 bits per digit. See Table 3 for address

information. Any unused bits are held to a logic “0” during a

read and ignored during a write operation.

NOTE:

1. 4.94304MHz is not available over military temperature range.

Alarm Compare RAM

On the chip are 51 bits of Alarm Compare RAM grouped into

words of different lengths. These are used to store the time,

ranging from 10ths of seconds to years, for comparison to the

real-time counters. Each counter has a corresponding RAM

word. In the Alarm Mode an interrupt is generated when the

current time is equal to the alarm time. The RAM contents are

compared to the counters on a word by word basis. If a

comparison to a particular counter is unnecessary, then the

appropriate ‘M’ bit in Compare RAM should be set to logic “1”.

The ‘M’ bit, referring to Mask bit, causes a particular RAM

word to be masked off or ignored during a compare. Table 3

shows addresses and Mask bit information.

Periodic Interrupts

The interrupt output can be programmed for 6 periodic

signals: 100Hz, 10Hz, once per second, once per minute,

once per hour, or once per day. The 100Hz and 10Hz

interrupts have instantaneous errors of

±2.5% and ±0.15%

respectively. This is because non-integer divider circuitry is

used to generate these signals from the crystal frequency,

which is a power of 2. The time average of these errors over

a 1 second period, however, is zero. Consequently, the

100Hz or 10Hz interrupts are not suitable as an aid in tuning

the oscillator; the 1 second interrupt must be used instead.

See General Notes, Note 6.

The periodic interrupts can occur concurrently and in

addition to alarm interrupts. The periodic interrupts are

controlled by bits in the interrupt mask register, and are

enabled by setting the appropriate bit to a “1” as shown in

Table 4. Bits D1 through D6 in the mask register, in

conjunction with bits D1 through D6 of the status register,

control the generation of interrupts according to Figure 5.

0

1

1

1

1

0F

RAM-day of week

M

-

-

-

-

0 - 6

1

0

0

0

0

10

Interrupt Status and Mask

Register

+

1

0

0

0

1

11

Command register

-

-

NOTES:

Addresses 10010 to 11111 (12h to 1Fh) are unused.

‘+’ Unused bit for interrupt Mask Register, MSB bit for interrupt Status Register.

‘-’ Indicates unused bits.

‘

‘M’ Alarm compare for particular counter will be enabled if bit is set to logic “0”.

TABLE 3. ADDRESS CODES AND FUNCTIONS (Continued)

ADDRESS

FUNCTION

DATA

VALUE

A4

A3

A2

A1

A0

HEX

D7

D6

D5

D4

D3

D2

D1

D0

TABLE 4. INTERRUPT AND STATUS REGISTERS FORMAT

INTERRUPT MASK REGISTER ADDRESS (10000b, 10h) WRITE-ONLY

D7

D6

D5

D4

D3

D2

D1

D0

NOT USED

DAY

HOUR

MIN

SEC

1/10 SEC

1/100 SEC

ALARM

←

Periodic Interrupt Mask Bits

→

Alarm/Compare Mask Bit

INTERRUPT STATUS REGISTER ADDRESS (10000b, 10h) READ-ONLY

D7

D6

D5

D4

D3

D2

D1

D0

GLOBAL INTERRUPT

DAY

HOUR

MIN

SEC

1/10 SEC

1/100 SEC

ALARM

Periodic and Alarm Flags

←

Periodic Interrupt Flags

→

Alarm Compare Flag

ICM7170