Table of Contents
Advertisement
9.6.3
Real-Time Clock (RTC) and Nonvolatile Memory
The signal PR_STROBE controls U17's output drivers. Under normal operation,
the outputs are tristated and resistors R148-R154 pull the outputs low.
PR_STROBE is driven low to turn on the output drivers of U17. Signals
PR_DOT0-PR_DOT6 (pins Q1-Q7) drive the 7 print dots of the optional printer.
PR_STROBE pulses all 7 of the dot lines for a specific time period (see also
Printer Interface, Section 6.14). When the CPU is first powered on,
PR_STROBE is in a tristate condition. R123 assures that U17 does not
accidentally turn on the printer head dots until required to. Pin Q8 (RTC_IO)
and an input bit of the external input port are connected. They work as a pair to
create a bidirectional bit for communicating with the RTC (see also "Real-Time
Clock and Nonvolatile Memory").
Both bytes of external output port (that is, U18 and U17) must be written to at
the same time. The upper byte of the output port (U17) cannot be written to
independently (see also "Address Decoding").
The Real-Time Clock Circuit is illustrated in Figure 9-21.
VCC
TP76
TEST
CR23
C114
1N914
.1uF
R163
3.32K
BT1
3V
Figure 9-21: Real-Time Clock Circuit
The RTC has two functions: (1) it provides nonvolatile 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-20PA printer. The N-20PA does not require or
use the RTC; it is disabled via software.
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 onto it from the RTC.
REAL TIME CLOCK
CR22
1N914
U29
16
VCC
SCLK
3
X1
I/O
5
X2
8
GND
RST
Y1
DS1202S
32.768KHz
Technical Supplement
TP86
TEST
RTC_CLK
14
12
RTC_IO
9
RTC_RST
TP85
TEST
TP87
TEST
9-19
Advertisement
Table of Contents
