In recent years, with the development of modern motor control technology and the booming market for motor drive systems, Analog Devices (AD) has launched the ADMCxx series of embedded DSPs for motor control. Among them, the ADMC401 is a high-end product suitable for high-precision applications such as industrial control and machine tool control. Currently, a number of documents discuss the application of the ADMC401 in electrical drives [1-3], but they all focus on introducing the transmission system or motor control algorithms, without systematically introducing the principles and characteristics of the ADMC401 from a chip perspective. The ADMC401 has a complete set of peripheral control interfaces and rich motor control peripheral circuits, combining the high-speed computing power of the DSP with the control capabilities of peripheral circuits, enabling motor control in a highly integrated environment. This article will focus on explaining the principles and characteristics of the ADMC401 and introduce its application in industrial control.
1ADMC401 Architecture
The architecture of the ADMC401 is shown in Figure 1. It mainly consists of a DSP core, memory space, and motor control peripheral circuits. For fully digital, high-performance motor control, the most distinctive motor control peripheral circuits of the ADMC401 are its on-chip analog-to-digital converter, pulse width modulation unit, and photoelectric encoder interface unit.
1.1 DSP Core and Storage Space
The DSP core is the "brain" of the ADMC401, and it is based on the 26MIPS fixed-point ADSP-2171 chip. The ADSP-2171 chip is a member of the ADSP-21xx family of Analog Devices, and its flexible architecture and complete instruction set allow the processor to execute multiple functions in parallel [4]. The ADMC401 is endowed with several system-level features of the ADSP-2171, such as memory mapping, interrupt system, and low-power operation.
The ADMC401's DSP core contains three computation units, two data address generators, and a program sequencer. Each computation unit includes an arithmetic logic unit (ALU), a multiply-accumulate (MAC), and a bucket shifter.
The ADMC401 has 2K×24bit of on-chip program memory RAM, 2K×24bit of on-chip program memory ROM, and 1K×16bit of data memory RAM. Furthermore, the ADMC401 can be expanded to 14K×24bit of program memory and 13K×16bit of data memory via external address and data buses.
1.2 Analog-to-Digital Conversion (ADC) System
The ADC system plays a crucial role in motor control. It is the "eye" of the controller, enabling the controller to monitor and regulate motor operation. The ADMC401 contains a fast, high-precision, multi-input ADC system with highly flexible operating modes. Its structural diagram is shown in Figure 2.
The ADMC401's ADC system has eight dedicated analog signal inputs. All signals are converted within 2μs by a 12-bit pipelined-flash analog-to-digital converter (ADC). The entire system operates at one-quarter of the system clock frequency, and the input analog voltage amplitude can reach 4V (peak-to-peak). The eight inputs are divided into two groups: VIN0~VIN3 and VIN4~VIN7. Each group has a dedicated input terminal that connects to the inverting input of a sample-and-hold amplifier, biasing the analog input to the normal input range of the ADC core.
The ADMC401's ADC system has two operating modes: synchronous sampling mode and sequential sampling mode. In synchronous sampling mode, VIN0 and VIN4, VIN1 and VIN5, VIN2 and VIN6, and VIN3 and VIN7 form four pairs of dual-channel synchronous sampling inputs, with each pair of analog signals being sampled and held synchronously. In sequential sampling mode, the eight analog signals are sampled and held one by one within one ADC clock cycle (or four DSP clock cycles).
This ADC system has two start-up modes: internal command start-up mode and external command start-up mode. Internal command start-up initiates A/D conversion on the rising edge of the PWM synchronization pulse (PWMSYNC); external command start-up initiates A/D conversion when the rising edge of the CONVST pin appears. The two start-up modes can be switched by setting the value of the control register.
The ADC system has two additional modes—bias correction mode and gain correction mode—used to correct the system's bias and gain, thereby increasing the overall system accuracy.
