DSP subsystem can make the mobile phone signal processing software effectively executed and flexibly applied. The block diagram is shown in Figure 4.
There are many kinds of DSP cores. For example; ADSP oak -2 18X, etc. Take OAK as an example to briefly introduce it. OAK core includes a 16-bit (data and program) fixed-point DSP with four 36-bit accumulators, a powerful word bit processing unit and a nested interrupt deep subroutine stack.
On-chip 16-bit data random access memory with a capacity of 4K.
When the processor stops at a program breakpoint, the Intelligent Debugging Interface (SDI) has access to all contents of the processor and all address spaces accessible by the controller. Through JTAG synchronous serial connection, the information is then sent to the computer host for display.
The address space accessible by DSP is controlled by OAK memory management unit (MMU). MMU is responsible for providing chip selection, controlling wait state and data width for all data access required by OAK kernel.
MMU manages DSP state changes; Work to sleep is realized by DSP software, and sleep to wake up is realized by interrupt.
Interrupt control register is a memory map, which hides and clears interrupts and configures the mapping between interrupt sources and interrupt signals (NMI, INT0, INT 1, INT2) generated by DSP.
There are five possible interrupt sources for DSP; Interrupt generated by ARM kernel, RX processing request (processing received RF signal samples), PCM I/F request (reading and writing voice signal samples), TDMA frame header tag, voice frame codec request.
According to GSM- 1C, some DSP resources (at least 1K program ram, 0. 5K data RAM (about 10MIPS computing power) can be used for special user programs.
DSP embedded code operation should realize speech coding and decoding, channel coding and decoding, encryption and decryption, burst generation and adjustment, power detection and so on. The main processing steps are shown in the figure;
DSP subsystem is the mapping of the internal and external settable address space of ARM7 kernel. In the address space inside ARM, the static position is reserved for DSP configuration, which is used for DSP state and information exchange of flow control. ARM gives two base addresses in the external address space, one for ROM for DSP to transfer the code from the external storage space to the internal program memory, and the other for ram as the storage space when DSP works. The MMU unit of ARM enables DSP to exchange data with external devices at high speed through DMA (Direct Memory Access) mechanism, and at the same time reduces the occupation of CPU resources during data exchange.
2。 DSP peripheral
DSP peripherals are mapped as memories or used as user-definable register interfaces of DSP, as shown in the following figure;
PCM I/F part processes audio data stream for DSP system; On the transmission path, it is responsible for transmitting audio sampling signals from the audio front-end or DAI port; In the receiving path, it transmits the decompressed audio sampling signal to the audio front-end or DAI port.
DSP RF port processes RF data stream for DSP subsystem; In the transmission path, it transmits the stored symbols to the digital GMSK modulator; In the receiving path, it stores the IQ signal sent from RX ADC until the DSP finishes processing.
Hard-wired coprocessor lightens the processing burden of DSP, which undertakes part of GSM signal processing that general DSP structure is not good at, and is also responsible for part of cryptographic algorithm processing and Viterbi decoding.