Datasheet

MC68HC908GP20 — Rev 2.1 Advance Information
Freescale Semiconductor 229
16.4 Port B
Port B is an 8-bit special-function port that shares all eight of its pins with
the analog-to-digital converter (ADC) module.
16.4.1 Port B Data Register
The port B data register (PTB) contains a data latch for each of the eight
port pins.
PTB7–PTB0 — Port B Data Bits
These read/write bits are software-programmable. Data direction of
each port B pin is under the control of the corresponding bit in data
direction register B. Reset has no effect on port B data.
AD7–AD0 — Analog-to-Digital Input Bits
AD7–AD0 are pins used for the input channels to the analog-to-digital
converter module. The channel select bits in the ADC status and
control register define which port B pin will be used as an ADC input
and overrides any control from the port I/O logic by forcing that pin as
the input to the analog circuitry.
NOTE: Care must be taken when reading port B while applying analog voltages
to AD7–AD0 pins. If the appropriate ADC channel is not enabled,
excessive current drain may occur if analog voltages are applied to the
PTBx/ADx pin, while PTB is read as a digital input. Those ports not
selected as analog input channels are considered digital I/O ports.
Address: $0001
Bit 7654321Bit 0
Read:
PTB7 PTB6 PTB5 PTB4 PTB3 PTB2 PTB1 PTB0
Write:
Reset: Unaffected by reset
Alternate
Function:
AD7 AD6 AD5 AD4 AD3 AD2 AD1 AD0
Figure 16-6. Port B Data Register (PTB)