M41T94 Serial real-time clock with 44 bytes NVRAM and reset Features Counters for tenths/hundredths of seconds, seconds, minutes, hours, day, date, month, year, and century 32 KHz crystal oscillator integrating load 16 capacitance (12.5 pF) providing exceptional oscillator stability and high crystal series 1 resistance operation Serial peripheral interface (2 MHz SPI) Ultralow battery supply current of 500 nA (max) SO16 (MQ) 2.7 to 5.5 V operating voltage 2.5 to 5.5 V oscillator operating voltage Battery low flag Automatic switchover and deselect circuitry 44 bytes of general purpose RAM Programmable alarm and interrupt function (valid even during battery backup mode) SNAPHAT (SH) Accurate programmable watchdog timer (from battery & crystal 62.5 ms to 128 s) Microprocessor power-on reset Choice of power-fail deselect voltages (V = 2.7 to 5.5 V): CC THS = V 2.55 V V 2.70 V SS PFD THS = V 4.20 V V 4.50 V CC PFD 28 Packaging includes a 28-lead SOIC and 1 SNAPHAT top (to be ordered separately) or 16-lead SOIC 28-lead SOIC package provides direct SOH28 (MH) connection for a SNAPHAT top which contains the battery and crystal RoHS compliant Lead-free second level interconnect January 2009 Rev 6 1/41 www.st.com 1Contents M41T94 Contents 1 Description . 6 2 Signal description 11 2.1 Serial data output (SDO) . 11 2.2 Serial data input (SDI) . 11 2.3 Serial clock (SCL) . 11 2.4 Chip enable (E) 11 3 Operation . 12 3.1 SPI bus characteristics . 13 3.2 Read and write cycles . 15 3.3 Data retention mode . 15 4 Clock operations . 17 4.1 Power-down time-stamp 17 4.2 Clock registers . 17 4.3 Setting alarm clock registers 19 4.4 Watchdog timer 21 4.5 Square wave output . 21 4.6 Power-on reset . 22 4.7 Reset inputs (RSTIN1 & RSTIN2) 22 4.8 Calibrating the clock . 23 4.9 Century bit 25 4.10 Output driver pin 26 4.11 Battery low warning . 26 4.12 t bit . 26 REC 4.13 Initial power-on defaults 27 5 Maximum ratings . 28 6 DC and AC parameters 29 7 Package mechanical data 32 2/41