AN2434 Freescale Semiconductor / Motorola, AN2434 Datasheet - Page 9

• Manufacturer Part Number: AN2434
• Description: Input/Output (I/O) Pin Drivers
• Manufacturer: Freescale Semiconductor / Motorola

Pullup Current
The pullup current was measured by a current meter in series between the input pin and a variable power
supply. With the power supply set to 5.0 Vdc, there is a slight leakage current into the input pin. This is
caused because there is a small voltage drop between the MCU’s power supply and the input pin. At
approximately 4.9 Vdc, the current is 0 µA (as expected). As the variable voltage is slowly reduced to 0
V, the current was measured and recorded.
current observed is approximately 57 µA.
It should be noted that the pullup mechanism is a weak semiconductor device and not a physical resistor
so the I-V characteristic is not linear, as would be expected for a pure resistor.
Output Characteristics
In
Figure
capable of forcing the intended level. Freescale Semiconductor specifies a maximum allowed current that
will still result in an output voltage that meets the specified limits for a logic 1 or logic 0. On some devices
(such as M68HC08 MCUs), different ports may have different source and sink current capability. On the
HC(S)12 Families, many ports have similar drive capabilities (with the exception of the motor controller
ports on the HC9S12H Family). This application note includes graphs of output voltage versus current for
conditions exceeding the specified standard load.
In the manufacturer specification, I/O drivers are characterized by the voltages observed at specified
maximum current levels for both the high and low logic levels. The test results for V
Freescale Semiconductor
Figure
12, port A0 pin is driven to a logic 0. When a heavy load (low impedance) is applied, the pin is not
7,
Figure
8, and
6
5
4
3
2
1
0
0
Figure 6. Port A — Pullup Current versus Voltage
Figure
10
11, the port A0 pin is driven to a logic 1. In
Input/Output (I/O) Pin Drivers, Rev. 0
20
Figure 6
30
CURRENT (µA)
summarizes the results. The maximum pullup
40
50
60
Figure
70
9,
OH
Figure
versus source
Pullup Current
10, and
9