Program Counter Low Register
To provide additional program control functions, the low byte of the Program Counter is made ac-
cessible to programmers by locating it within the Special Purpose area of the Data Memory. By ma-
nipulating this register, direct jumps to other program locations are easily implemented. Loading a
value directly into this PCL register will cause a jump to the specified Program Memory location,
however, as the register is only 8-bit wide, only jumps within the current Program Memory page
are permitted. When such operations are used, note that a dummy cycle will be inserted.
Look-up Table Registers
These special function registers are used to control operation of the look-up table which is stored
in the Program Memory. TBLP is the table low byte pointer and indicates the location where the ta-
ble data is located. Its value must be setup before any table read commands are executed. Its
value can be changed, for example using the
instructions, allowing for easy table
data pointing and reading. TBHP is the table high byte pointer but is only available in the
HT49RU80/HT49CU80 devices. TBLH is the location where the high order byte of the table data is
stored after a table read data instruction has been executed. Note that the lower order table data
byte is transferred to a user defined location.
Real Time Clock Control Register
The RTCC register controls several internal functions one of which is the Real Time Clock (RTC)
Interrupt, whose function is to provide an internal interrupt signal at regular fixed intervals. The driv-
ing clock for the RTC interrupt comes from the internal clock source, known as f
. which is then fur-
ther divided to give longer time values, which in turn generates the interrupt signal. The value of
this division ratio is determined by the value programmed into bits 2~0, known as RT2~RT0, of the
RTCC register. By writing a value directly into these RTCC register bits, time-out values from 2
to 2
can be generated. The RTCC register also controls the quick start-up function of the RTC
oscillator. This oscillator which has a fixed frequency of 32768Hz can be made to start-up at a
quicker rate by setting bit 4, known as the QOSC bit to
This bit will be set to a
value when
the device is powered on, however, as some extra power is consumed, the QOSC bit should be
set to
after about 2 seconds to reduce power consumption. A further internal function under the
control of the RTCC register is the Low Voltage Detector. This function can be enabled by setting
bit 3, known as the LVDC bit, to
When the power supply voltage falls below a certain VLVD
value, as specified in the DC characteristics, bit 5, which is a read only bit and known as LVDO, will
be set to
This bit will remain at a
value if the power supply voltage is above the specified
level. Note that bits 6 and 7 of the RTCC register are not used.
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