Mallinckrodt Nellcor N-20 Service Manual page 57

9.10.11 Real-Time Clock (RTC) and Non-Volatile Memory
The real-time clock circuit is illustrated in Figure 9-20.
The RTC has two functions: (1) it provides non-volatile memory that is used to remember whether the
printer should be enabled at power on, and (2) to keep track of time and date for the N-20P printer.
The N-20 does not require or use the RTC; it is disabled via software.
TP76
TEST
C114
.1uF
The RTC chip U29 uses a 3-wire synchronous serial interface to communicate with the CPU. The
CPU brings signal RTC_RST high to activate communication with the RTC. RTC_CLK clocks data
into and out of the RTC chip. RTC_IO is the bidirectional communication data bit. The CPU drives
RTC_IO when writing data and commands to the RTC. The CPU tristates RTC_IO and then reads
data back on it from the RTC.
Crystal Y1 provides an accurate 32.768 KHz clock input whenever the time keeping circuitry of U29
is activated. The CPU enables the timekeeping function only when an optional printer is installed. If
no printer is installed, the CPU switches off timekeeping, thereby extending battery life. Also, with no
printer installed, the RTC clock is used only during diagnostic testing to verify the CPU clock timing.
The lithium battery BT1 and diodes CR22 and CR23 provide the power switch over and constant
power needed to keep the time and RAM data while the unit is not in use. Whenever the unit is
powered on, Vcc is at 5 V and U29 is powered via CR22. CR 23 is reverse biased because BT1 at 3 V
is at a lower potential than Vcc. Whenever the unit is powered off, the potential between Vcc and
switched ground is 0 V, CR23 is forward biased, and U29 is powered by BT1. CR22 is reverse
biased, isolating BT1 from Vcc. This circuit design allows BT1 life of up to 5 years, typically, without
the unit being powered on.
U29 holds 24 bytes of RAM, which is used for non-volatile storage of CPU data.
9.10.12 Audio Output
The audio output circuit is illustrated in Figure 9-21.
REAL TIME CLOCK
VCC
CR22
1N914
16
3
CR23
5
8
1N914
Y1
32.768KHz
R163
3.32K
BT1
3V
Figure 9-20: Real-Time Clock Circuit
TP86
TEST
U29
RTC_CLK
14
VCC SCLK
12 RTC_IO
X1 I/O
X2
9 RTC_RST
GND RST
DS1202S
Technical Supplement
TP85
TEST
TP87
TEST
9-19