Philips CEM2100/00, Service Manual

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Page 54
5. I/O Ports
5.3 Port P2 (P22 to P20)
T
5CL8
5.3 Port P2 (P22 to P20)
Port P2 is a 3-bit input/output port.
It is also used as an external interrupt, a STOP mode release signal input, and low-frequency crystal oscillator con-
nection pins. When used as an input port or a secondary function pins, respective output latch (P2DR) should be set
to “1”.
During reset, the P2DR is initialized to “1”.
A low-frequency crystal oscillator (32.768 kHz) is connected to pins P21 (XTIN) and P22 (XTOUT) in the dual-
clock mode. In the single-clock mode, pins P21 and P22 can be used as normal input/output ports.
It is recommended that pin P20 should be used as an external interrupt input, a STOP mode release signal input, or
an input port. If it is used as an output port, the interrupt latch is set on the falling edge of the output pulse.
P2 port output latch (P2DR) and P2 port terminal input (P2PRD) are located on their respective address.
When read the output latch data, the P2DR should be read and when read the terminal input data, the P2PRD reg-
ister should be read. If a read instruction is executed for port P2, read data of bits 7 to 3 are unstable.
Figure 5-4 Port 2
Note: Port P20 is used as
STOP
pin. Therefore, when stop mode is started, OUTEN does not affect to P20, and P20 becomes
high-Z mode.
P2DR
(0002H)
R/W
7 6 5 4 3 2 1 0
P22
XTOUT
P21
XTIN
P20
INT5
STOP
(Initial value: **** *111)
P2PRD
(000CH)
Read only
P22 P21 P20
Output latch
Osc. enable
Output latch
D Q
P20 (INT5, STOP)
D Q
Output latch
D Q
P21 (XTIN)
P22 (XTOUT)
Data input (P20PRD)
Data input (P20)
Data output (P20)
Contorl input
Data input (P21PRD)
Output latch read (P21)
Data output (P21)
Data input (P22PRD)
Output latch read (P22)
Data output (P22)
STOP
OUTEN
XTEN
fs