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Technical Supplement
U14 is a standard 8K × 8 static RAM. Test point TP 43 is used during testing to disable the output.
The program that the CPU runs is stored in U15. U15 is a 16-bit wide output, one-time programmable
(OTP) EPROM. During 16-bit wide bus accesses, the CPU uses address line A0 for low/high byte
selection, and address line A0 is not used as a normal address line. The CPU can address only 64K × 8
bytes or 32K × 16 bytes. Pin A15 of U15 is tied low, always selecting the lower half of the EPROM.
Signal ROMEN is then used to enable the EPROM for the proper memory area.
9.10.10.1
Input Port
The input port circuit is illustrated in Figure 9-18.
U16 is the input port external to the CPU. The logic levels on the inputs (pins D1-D8) are output to
the CPU via the AD bus while EXINEN is strobed low. All of the user control buttons are input via
U16. Also, the battery type is sensed via U16; a high on signal BAT_TYPE signifies to the CPU that
rechargeable batteries are being used. If the optional printer head is in the home position, PR_HOME
will be a logic high.
Pin D8 (RTC_IO) and an output bit of the external output port are connected. They work as a pair to
create a bidirectional bit for communicating with the RTC (see Section 3.5.3, Real-Time Clock and
Non-Volatile Memory).
9.10.10.2
Output Port
The output port circuit is illustrated in Figure 9-19 (at the end of this section).
The output port external to the CPU consists of 2 octal D latches , U18 and U17; they function as a
single 16-bit output port. U18 is the lower byte (LSB) and U17 is the upper byte (MSB). The output
of U18 is always enabled. The output bits of U18 control: audio output, optional printer, RTC, and
display.
The signal PR_STROBE controls U17s 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"). 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 Non-Volatile Memory").
Both bytes of external output port (i.e., 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").
9-18
EX_IN INPUT PORT
U16
AD0
19
Q1
D1
AD1
18
Q2
D2
AD2
17
Q3
D3
AD3
16
Q4
D4
AD4
15
Q5
D5
AD5
14
Q6
D6
AD6
13
Q7
D7
AD7
12
Q8
D8
C
OC
EXINEN
74HC573
Figure 9-18: Input Port Circuit
2
GO_BTN
3
4
ON_OFF_BTN
DD_BTN
5
6
ADV_BTN
7
PR_HOME
BAT_TYPE
8
9
RTC_IO
11
VCC
1
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