dsp56800e Freescale Semiconductor, Inc, dsp56800e Datasheet - Page 244

• Manufacturer Part Number: dsp56800e
• Description: 16-bit Digital Signal Controller Core
• Manufacturer: Freescale Semiconductor, Inc

Program Controller
8.2.1.1
The operating mode (MB and MA) bits are used to select the operating mode and memory map. Their
initial values after reset are typically established by external mode select pins. After the chip leaves the
reset state, MB and MA can be changed under program control. Consult the specific DSP56800E device’s
user’s manual for more information about how these bits are established on reset and about their specific
effect on operation.
8.2.1.2
The external X memory (EX) bit can be used to configure the location of data memory. Typically, a
DSP56800E–based device has some quantity of on-chip data memory, which can be supplemented by
external data memory as needed. The EX bit can be used by a chip to select whether both on-chip and
external memories are used or whether all data memory accesses are sent to external memory.
The exact effect of the EX bit depends on the architecture of a given device. Consult the appropriate
device’s user’s manual for more information on the EX bit.
8.2.1.3
The saturation (SA) bit enables automatic saturation in the data ALU on 32-bit arithmetic results.
Normally, saturation occurs only when an accumulator is written to memory. When the SA bit is set,
saturation is performed on the results of all basic arithmetic operations, such as multiplication or addition,
before they are stored in an accumulator. This automatic saturation is useful for bit-exact DSC algorithms
that do not recognize or cannot take advantage of the extension registers that are available with each
accumulator. Automatic saturation is discussed in detail in Section 5.8.2, “MAC Output Limiter,” on
page 5-41. This bit is cleared by processor reset.
8.2.1.4
The rounding (R) bit selects the type of rounding that is used when RND, MACR, and other instructions
that round values are executed. When set, two’s-complement rounding (always round up) is used. When
cleared, convergent rounding is selected. The two rounding modes are discussed in Section 5.9,
“Rounding,” on page 5-43. This bit is cleared by processor reset.
8.2.1.5
The stop delay (SD) bit selects the amount of time it takes to wake up from stop mode. When the bit is set,
the processor exits quickly from stop mode; when the bit is cleared, a delay is inserted before the processor
exits stop mode. A long wake-up time can be useful to allow a crystal oscillator to settle before resuming
instruction execution. The exact length of the delay depends on the particular DSP56800E device that is
being used. Consult the device’s user’s manual for more information. This bit is cleared by processor reset.
8.2.1.6
The X or P memory (XP) bit is used to select the memory space—program or data—from which
instructions are fetched. In most cases, this bit is cleared and instructions are fetched from program
memory. On devices that support execution from both memory spaces, this bit can be set so that
instructions are fetched from data memory. Refer to Section 8.6, “Executing Programs from Data
Memory,” for more information on executing programs from data memory. This bit is cleared by processor
reset.
8-6
Operating Mode (MA and MB)—Bits 0–1
External X Memory (EX)—Bit 3
Saturation (SA)—Bit 4
Rounding (R)—Bit 5
Stop Delay (SD)—Bit 6
X or P Memory (XP)—Bit 7
DSP56800E Core Reference Manual
Freescale Semiconductor