Analog Devices ADSP-21020 - Manual (Page 4)

ADSP-21020
REV. C –4–
in a specified register, either before (premodify) or after
(postmodify) the access. To implement automatic modulo
addressing for circular buffers, the ADSP-21020 provides buffer
length registers that can be associated with each pointer. Base
values for pointers allow circular buffers to be placed at arbitrary
locations. Each DAG register has an alternate register that can
be activated for fast context switching.
The program sequencer supplies instruction addresses to
program memory. It controls loop iterations and evaluates
conditional instructions. To execute looped code with zero
overhead, the ADSP-21020 maintains an internal loop counter
and loop stack. No explicit jump or decrement instructions are
required to maintain the loop.
The ADSP-21020 derives its high clock rate from pipelined
fetch, decode and execute cycles. Approximately 70% of the
machine cycle is available for memory accesses; consequently,
ADSP-21020 systems can be built using slower and therefore
less expensive memory chips.
Instruction Cache
The program sequencer includes a high performance, selective
instruction cache that enables three-bus operation for fetching
an instruction and two data values. This two-way, set-associative
cache holds 32 instructions. The cache is selective—only the
instructions whose fetches conflict with program memory data
accesses are cached, so the ADSP-21020 can perform a program
memory data access and can execute the corresponding instruction
in the same cycle. The program sequencer fetches the instruction
from the cache instead of from program memory, enabling the
ADSP-21020 to simultaneously access data in both program
memory and data memory.
Context Switching
Many of the ADSP-21020’s registers have alternate register sets
that can be activated during interrupt servicing to facilitate a fast
context switch. The data registers in the register file, DAG
registers and the multiplier result register all have alternate sets.
Registers active at reset are called primary registers; the others
are called alternate registers. Bits in the MODE1 control register
determine which registers are active at any particular time.
The primary/alternate select bits for each half of the register file
(top eight or bottom eight registers) are independent. Likewise,
the top four and bottom four register sets in each DAG have
independent primary/ alternate select bits. This scheme allows
passing of data between contexts.
Interrupts
The ADSP-21020 has four external hardware interrupts, nine
internally generated interrupts, and eight software interrupts.
For the external interrupts and the internal timer interrupt, the
ADSP-21020 automatically stacks the arithmetic status and
mode (MODE1) registers when servicing the interrupt, allowing
five nesting levels of fast service for these interrupts.
An interrupt can occur at any time while the ADSP-21020 is
executing a program. Internal events that generate interrupts
include arithmetic exceptions, which allow for fast trap handling
and recovery.
Timer
The programmable interval timer provides periodic interrupt
generation. When enabled, the timer decrements a 32-bit count
register every cycle. When this count register reaches zero, the
ADSP-21020 generates an interrupt and asserts its TIMEXP
output. The count register is automatically reloaded from a
32-bit period register and the count resumes immediately.
System Interface
Figure 2 shows an ADSP-21020 basic system configuration.
The external memory interface supports memory-mapped
peripherals and slower memory with a user-defined combination
of programmable wait states and hardware acknowledge signals.
Both the program memory and data memory interfaces support
addressing of page-mode DRAMs.
The ADSP-21020’s internal functions are supported by four
internal buses: the program memory address (PMA) and data
memory address (DMA) buses are used for addresses associated
with program and data memory. The program memory data
(PMD) and data memory data (DMD) buses are used for data
associated with the two memory spaces. These buses are
extended off chip. Four data memory select (DMS) signals
select one of four user-configurable banks of data memory.
Similarly, two program memory select (PMS) signals select
between two user-configurable banks of program memory. All
banks are independently programmable for 0-7 wait states.
The PX registers permit passing data between program memory
and data memory spaces. They provide a bridge between the
48-bit PMD bus and the 40-bit DMD bus or between the 40-bit
register file and the PMD bus.
The PMA bus is 24 bits wide allowing direct access of up to
16M words of mixed instruction code and data. The PMD is 48
bits wide to accommodate the 48-bit instruction width. For
access of 40-bit data the lower 8 bits are unused. For access of
32-bit data the lower 16 bits are ignored.
The DMA bus is 32 bits wide allowing direct access of up to 4
Gigawords of data. The DMD bus is 40 bits wide. For 32-bit
data, the lower 8 bits are unused. The DMD bus provides a
path for the contents of any register in the processor to be
transferred to any other register or to any external data memory
location in a single cycle. The data memory address comes from
one of two sources: an absolute value specified in the instruction
code (direct addressing) or the output of a data address
generator (indirect addressing).
External devices can gain control of the processor’s memory
buses from the ADSP-21020 by means of the bus request/grant
signals (BR and BG). To grant its buses in response to a bus
request, the ADSP-21020 halts internal operations and places
its program and data memory interfaces in a high impedance
state. In addition, three-state controls (DMTS and PMTS)
allow an external device to place either the program or data
memory interface in a high impedance state without affecting
the other interface and without halting the ADSP-21020 unless
it requires a memory access from the affected interface. The
three-state controls make it easy for an external cache controller
to hold the ADSP-21020 off the bus while it updates an external
cache memory.
JTAG Test and Emulation Support
The ADSP-21020 implements the boundary scan testing
provisions specified by IEEE Standard 1149.1 of the Joint
Testing Action Group (JTAG). The ADSP-21020’s test
access port and on-chip JTAG circuitry is fully compliant with
the IEEE 1149.1 specification. The test access port enables
boundary scan testing of circuitry connected to the
ADSP-21020’s I/O pins.