Section 17 Real-Time Clock - Mitsubishi DS907x SIP User Manual

USER'S GUIDE
SECTION 17: REAL–TIME CLOCK
Many user applications require a time–of–day clock.
For this reason, all Secure Microcontroller modules
have real–time clock (RTC) options. These include the
DS5000T DIP and the DS2250T, DS2251T, and
DS2252T SIMMs. In addition, users of the monolithic
microprocessor chips will frequently connect to a Dallas
Semiconductor RTC. There are two types of clock used
in Dallas modules. These are the DS1215 Phantom
Time Chip and the generally superior DS1283 Watch-
dog Timekeeper Chip.
This section is intended to provide only a brief overview
of the RTCs used on the time–microcontroller modules.
For a more detailed description, please consult the Dal-
las Semiconductor Timekeeping & NV RAM Data Book.
DS1215 PHANTOM TIME CHIP
The DS5000T and DS2250T microcontrollers use a
custom device similar to the DS1215 Phantom Time
Chip. This clock gives permanently powered time–of–
day monitoring. The clock runs from an internal 32 KHz
crystal and is generally independent of the microcontrol-
ler. It provides time of day information including 0.01
second, seconds, minutes, hours, day, date, month and
year. The register format is shown below. The DS1215
keeps time to two minutes per month accuracy. It offers
a complete representation of time and calendar in a con-
venient BCD format. It does not provide any interrupt
DS5000T/DS2250T FUNCTIONAL BLOCK DIAGRAM Figure 17–1
050396 143/173
DS5000T OR DS2250T
CE2
DS5000
CPU
BA2
BA0
BD7
capability. These functions are provided in the DS1283
type clock that is used in the DS2251T and DS2252T.
The RTC used in the DS5000T/DS2250T is transparent
to the memory map. Figure 17–1 shows a functional
block diagram of the interconnection between the
DS5000FP and RTC used on the DS5000T/DS2250T.
It is fundamentally a serial device that resides on the
address bus. To access the clock, the user must set the
ECE2 bit at MCON.2 to a logic 1. This will cause all
MOVX instructions to access CE2 instead of CE1.
Once ECE2 is set, the Byte–wide Address bit 2 serves
as a write enable and Address bit 0 serves as the data
input. Bit 7 of the Byte–wide Data bus serves as the
data output. Notice that the read/write line is not used.
For each CE2 access, the DS1215 will watch the value
of A0 on the Byte–wide bus for a particular 64–bit secu-
rity pattern. This pattern checking prevents accidentally
invoking the clock. Since these must be write opera-
tions, A2 must be a logic 0 for each write. The clock will
take no action unless the 64 pattern bits are written in
the correct order.
comparator to start over. Thus the users must "really"
intend to communicate with the DS1215. Once the
security pattern is written, the next 64 bits are time of
day and calendar functions. Thus 128 read/writes are
required for any time of day access. Data is written
using BA0 and read using BD7. Thus the address actu-
ally writes data, but data is read normally using one bit.
CE1
RTC
WE
D
Q
144
Any error causes the pattern