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AD5241 Arkusz danych(PDF) 7 Page - Analog Devices |
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AD5241 Arkusz danych(HTML) 7 Page - Analog Devices |
7 / 20 page REV. 0 AD5232 –7– OPERATIONAL OVERVIEW The AD5232 digital potentiometer is designed to operate as a true variable resistor replacement device for analog signals that remain within the terminal voltage range of VSS < VTERM < VDD. The basic voltage range is limited to a |VDD – VSS| < 5.5 V. The digital potentiometer wiper position is determined by the RDAC register contents. The RDAC register acts as a scratch pad, register allowing as many value changes as necessary to place the potentiometer wiper in the correct position. The scratch pad register can be programmed with any position value using the standard SPI serial interface mode by loading the complete representative data word. Once a desirable position is found, this value can be saved into a corresponding EEMEM register. Thereafter the wiper position will always be set at that position for any future ON-OFF-ON power supply sequence. The EEMEM save process takes approximately 25 ms, during this time the shift register is locked preventing any changes from taking place. The RDY pin indicates the completion of this EEMEM save. SCRATCH PAD AND EEMEM PROGRAMMING The scratch pad register (RDAC register) directly controls the position of the digital potentiometer wiper. When the scratch pad register is loaded with all zeros, the wiper will be connected to the B-Terminal of the variable resistor. When the scratch pad register is loaded with midscale code (1/2 of full-scale position), the wiper will be connected to the middle of the variable resis- tor. And when the scratch pad is loaded with full-scale code, all 1s, the wiper will connect to the A-Terminal. Since the scratch pad register is a standard logic register, there is no restriction on the number of changes allowed. The EEMEM registers have a program erase/write cycle limitation described in the Flash/ EEMEM Reliability section. BASIC OPERATION The basic mode of setting the variable resistor wiper position (programming the scratch pad register) is accomplished by loading the serial data input register with the command instruc- tion #11, which includes the desired wiper position data. When the desired wiper position is found, the user loads the serial data input register with the command instruction #2, which copies the desired wiper position data into the corresponding nonvola- tile EEMEM register. After 25 ms the wiper position will be permanently stored in the corresponding nonvolatile EEMEM location. Table I provides an application-programming example listing the sequence of serial data input (SDI) words and the corresponding serial data output appearing at the SDO pin in hexadecimal format. At system power-on, the scratch pad register is refreshed with the value last saved in the EEMEM register. The factory preset EEMEM value is midscale. The scratch pad (wiper) register can be refreshed with the current contents of the nonvolatile EEMEM register under hardware control by pulsing the PR pin. Table I. Set Two Digital POTs to Independent Data Values then Save Wiper Positions in Corresponding Nonvolatile EEMEM Registers SDI SDO Action B040H XXXXH Loads 40H data into RDAC1 register, Wiper W1 moves to 1/4 full-scale position. 20xxH B040H Saves copy of RDAC1 register contents into corresponding EEMEM0 register. B180H 20xxH Loads 80H data into RDAC2 register, Wiper W2 moves to 1/2 full-scale position. 21xxH B180H Saves copy of RDAC2 register contents into corresponding EEMEM1 register. Be aware that the PR pulse first sets the wiper at midscale when brought to logic zero, and then on the positive transition to logic high, it reloads the DAC wiper register with the contents of EEMEM. Many additional advanced programming commands are available to simplify the variable resistor adjustment process. For example, the wiper position can be changed one step at a time by using the software-controlled Increment/Decrement instruction or, by 6 dB at a time, with the Shift Left/Right instruction command. Once an Increment, Decrement, or Shift command has been loaded into the shift register, subsequent CS strobes will repeat this command. This is useful for push-button control applications. See the Advanced Control Modes descrip- tion following Table I. A serial data output SDO pin is available for daisy chaining and for readout of the internal register contents. The serial input data register uses a 16-bit [instruction/address/data] WORD. EEMEM PROTECTION Write protect ( WP) disables any changes of the scratch pad register contents regardless of the software commands, except that the EEMEM setting can be refreshed using commands 8 and PR. Therefore, the write-protect (WP) pin provides a hard- ware EEMEM protection feature. Execute a NOP command before returning WP to logic high. DIGITAL INPUT/OUTPUT CONFIGURATION All digital inputs are ESD-protected high input impedance that can be driven directly from most digital sources. PR and WP, which are active at logic low, must be biased to VDD if they are not being used. No internal pull-up resistors are present on any digital input pins. The SDO and RDY pins are open-drain digital outputs where pull-up resistors are needed only if using these functions. A resistor value in the range of 1 k Ω to 10 kΩ optimizes the power and switching speed trade-off. |
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