1. Parallel interface
MAXIM-DALLAS DS12C887 series: including ds1302, ds1307, ds1337, pcf8563
2. Serial interface
1) I2C interface: PCF8563 (Phlilps), PCF8583 (Phlilps), RX8025 (EPSON): they built-in crystal vibration and the error is small.
Pretty good: DS1307 (MAXIM-DALLAS), RS5C372 (RICOH), Imitated model BL5372 (Domestic BELLING), S-35390 (Japan NSK), X1288 (Intersil), SD2000 series (Shenzhen Weifan Electronics Co. Ltd): they built-in crystal vibration and batteries. The size is much bigger.
2) Three-wire interface: DS1305, DS1302, HT1380, HT1381 (MAXIM-DALLAS): real time clock ic chip is a single chip microcomputer timer clock or independent clock which can be accessed by single chip microcomputer. It can be obtained from external expansion chips such as 1302, 1307, 12887, 3130, 12020, m41t81, 6902, 8025. There are parallel port and serial port. In accordance with your actural needs, there are with batteries or with external power supply. All these clocks used chips (32768Hz) without exception. This is because they used the same timing IC kernel, which with lower low-frequency power consumption, 1Hz timing precision implementation and much easier to correct. We made different improvements. For example, built-in the stable crystals, back-up power study, different interfaces study, calendar study, alarm function and so on.
3) There may be also that you mentioned (probably about the 430 series single chip microcomputer). Internal integration RTC module which requires 32768Hz. It can be timed independently. Even though the single chip microcomputer "sleeps", the time management is also be running. Low cost real-time solution makes you save money.