sx20ac Parallax, Inc., sx20ac Datasheet - Page 9

• Manufacturer Part Number: sx20ac
• Description: Configurable Communications Controllers With Ee/flash Program Memory, In-system Programming Capability And On-chip Debug
• Manufacturer: Parallax, Inc.

Parallax SX20AC/SX28AC
3.0
All models contain a 4-bit I/O port (Port A), an 8-bit I/O
port (Port B). The SX28 also contains a second 8-bit I/O
port (Port C). Port A provides symmetrical drive
capability. Each port has three associated 8-bit registers
(Direction, Data, TTL/CMOS Select, and Pull-Up Enable)
to configure each port pin as Hi-Z input or output, to
select TTL or CMOS voltage levels, and to enable/disable
the weak pull-up resistor. The upper four bits of the
3.1.
The three ports are memory-mapped into the data memory
address space. To the CPU, the three ports are available as
the RA, RB, and RC file registers at data memory
addresses 05h, 06h, and 07h, respectively. Writing to a
port data register sets the voltage levels of the
© Parallax Inc.
Output
PORT DESCRIPTIONS
Reading and Writing the Ports
Data Direction Registers:
0
RA, RB, RC
Hi-Z Input
1
Table 3-1: Port Configuration
TTL/CMOS Selected Registers:
CMOS
0
LVL_A, LVL_B, LVL_C
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registers associated with Port A are not used. The least
significant bit of the registers corresponds to the least
significant port pin. To access these registers, an
appropriate value must be written into the MODE
register. Upon power-up, all bits in these registers are
initialized to “1”. The associated registers allow for each
port bit to be individually configured under software
controls as shown below.
corresponding port pins that have been configured to
operate as outputs to a corresponding level, 1 = 5 V,
0 = 0 V. Reading from a register reads the voltage levels
of all port pins.
TTL
1
Figure 3-1
Port A Configuration
Enable
0
Pullup Enable Registers:
PLP_A, PLP_B, PLP_C
Rev 1.6 11/20/2006
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Disable
1