HT47C20 Holtek Semiconductor Inc, HT47C20 Datasheet - Page 28

• Manufacturer Part Number: HT47C20
• Description: 8-Bit Microcontroller
• Manufacturer: Holtek Semiconductor Inc

Input/output ports
There are 8-bit bidirectional input/output port
and 4-bit input port in the HT47C20, labeled PA
and PB which are mapped to the data memory
of [12H] and [14H] respectively. The high nibble
of the PA is NMOS output and input with
pull-high resisters. The low nibble of the PA can
be used for input/output or output operation by
selecting NMOS or CMOS output by mask op-
tion. Each bit on the PA can be configured as a
wake-up input, and the low nibble of the PA
with or without pull-high resistor by mask op-
tion. PB can only be used for input operation,
and each bit on the port can be configured with
pull high resistor by mask option. Both are for
the input op er ati on , thes e p or t s a re
non-latched, that is, the inputs should be ready
at the T2 rising edge of the instruction ²MOV A,
[m]² (m=12H or 14H). For PA output operation,
all data are latched and remain unchanged un-
til the output latch is rewritten.
PA0/BZ, PA1/BZ input/output port
28
When the structures of PA are open drain
NMOS type, it should be noted that, before
reading data from the pads, a ²1² should be
written to the related bits to disable the NMOS
device. That is done first before executing the
instruction ²MOV A, 0FFH² and ²MOV [12H],
A² to disable related NMOS device, and then
²MOV A, [12H]² to get stable data.
After chip reset, these input lines remain at a
high level or are left floating (by mask option).
Some instructions first input data and then fol-
low the output operations. For example, ²SET
[m].i², ²CLR [m].i², ²CPL [m]², ²CPLA [m]² read
the entire port states into the CPU, execute the
defined operations (bit-operation), and then
write the results back to the latches or to the ac-
cumulator. Each bit of the PA output latches
can not use these instruction, which may
change the input lines to output lines (when in-
put line is at low level).
January 18, 2000
HT47C20