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Technical Supplement
9.6.2.1 Input Port
9.6.2.2 Output Port
9-18
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. A0 is not used as a
normal address line. The CPU can only address 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.
The input port circuit is illustrated in Figure 9-19.
AD0
AD1
AD2
AD3
AD4
AD5
AD6
AD7
EXINEN
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 senses 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 6.3, Real-Time Clock and Non-Volatile Memory).
The output port circuit is illustrated in Figure 9-20, at the end of Section 9.
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.
EX_IN INPUT PORT
U16
19
2
Q1
D1
18
3
Q2
D2
17
4
Q3
D3
16
5
Q4
D4
15
6
Q5
D5
14
7
Q6
D6
13
8
Q7
D7
12
9
Q8
D8
11
C
1
OC
74HC573
Figure 9-19: Input Port Circuit
GO_BTN
ON_OFF_BTN
DD_BTN
ADV_BTN
PR_HOME
BAT_TYPE
RTC_IO
VCC
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