MC68HC05J3 Datasheet, PDF (34/92 Pages) Motorola, Inc – High-density Complementary Metal Oxide Semiconductor (HCMOS) Microcomputer Unit

4.5
Other port considerations
All output ports can emulate ‘open-drain’ outputs. This is achieved by writing a zero to the relevant
output port latch. By toggling the corresponding data direction bit, the port pin will either be an
output zero or tri-state (an input). This is shown diagrammatically in Figure 4-3.
When using a port pin as an ‘open-drain’ output, certain precautions must be taken in the user
software. If a read-modify-write instruction is used on a port where the ‘open-drain’ is assigned
and the pin at this time is programmed as an input, it will read it as a ‘one’. The read-modify-write
4
instruction will then write this ‘one’ into the output data latch on the next cycle. This would cause
the ‘open-drain’ pin not to output a ‘zero’ when desired.
Note: ‘Open-drain’ outputs should not be pulled above V_DD.
Read buffer output
(a)
A
Y
Data direction register bit DDRn
DDRn
1
1
0
0
(b)
1
1
0
0
A
Y
0
0
1
1

 Normal operation – tri state
0 tri state _
1 tri state ^
0
low 
1
—

 ‘Open-drain’
0
high _
1
high ^
V_DD
VDD
Px0
‘Open-drain’ output
(c)
DDRx, bit 0 = 0
Portx, bit 0 = 0
Figure 4-3 Port logic levels
DDRx, bit 0 = 0
Portx, bit 0 = 0
MOTOROLA
4-6
INPUT/OUTPUT PORTS
TPG
MC68HC05J3