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SA828 Datasheet(PDF) 5 Page - Mitel Networks Corporation |
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SA828 Datasheet(HTML) 5 Page - Mitel Networks Corporation |
5 / 14 page 5 SA828 Register R0 R1 R2 R3 R4 Comment Temporary register R0 Temporary register R1 Temporary register R2 Transfers control data Transfers initialisation data AD1 0 0 1 1 0 Power frequency range This sets the maximum power frequency that can be carried within the PWM output waveforms. This would normally be set to a value to prevent the motor system being operated outside its design parameters. Pulse delay time ('underlap') For each phase of the PWM cycle there are two control signals, one for the top switch connected to the positive inverter DC supply and one for the bottom switch connected to the negative inverter DC supply. In theory, the states of these two switches are always complementary. However, due to the finite and non-equal turn-on and turn- off times of power devices, it is desirable when changing the state of the output pair, to provide a short delay time during which both outputs are off in order to avoid a short circuit through the switching elements. Pulse deletion time A pure PWM sequence produces pulses which can vary in width between 0% and 100% of the duty cycle. Therefore, in theory, pulse widths can become infinitesimally narrow. In practice this causes problems in the power switches due to storage effects and therefore a minimum pulse width time is required. All pulses shorter than the minimum specified are deleted. Counter reset This facility allows the internal power frequency counter of the SA828 to be set to zero, disabling the normal frequency control and giving a 50% output duty cycle. Initialisation Register Programming The initialisation register data is loaded in 8-bit segments into the three 8-bit temporary registers R0-R2. When all the initialisation data has been loaded into these registers it is transferred into the 24-bit initialisation register by writing to the dummy register R4. AD2 0 0 0 0 1 AD0 0 1 0 1 0 Table 3 SA828 register addressing Initialisation Register Function The 24-bit initialisation register contains parameters which, under normal operation, will be defined during the power-up sequence. These parameters are particular to the drive circuitry used, and therefore changing these parameters during a PWM cycle is not recommended. Information in this register should only be modified while RST is active (i.e. low) so that the PWM outputs are inhibited (low) during the updating process. The parameters set in the initialisation register are as follows: Carrier frequency Low carrier frequencies reduce switching losses whereas high carrier frequencies increase waveform resolution and can allow ultrasonic operation. Carrier frequency selection The carrier frequency is a function of the externally applied clock frequency and a division ratio n, determined by the 3-bit CFS word set during initialisation. The values of n are selected as shown in Table 4. The carrier frequency, fCARR, is then given by: CFS word Value of n 000 1 001 2 010 4 011 8 100 16 101 32 Table 4 Values of clock division ratio n FRS2 FRS1 FRS0 X X CFS2 CFS2 CFS2 FREQUENCY RANGE SELECT WORD FRS2 = MSB FRS0 = LSB DON’T CARE CARRIER FREQUENCY SELECT WORD CFS2 = MSB CFS0 = LSB Fig. 6 Temporary register R1 where k = clock frequency and n = 1, 2, 4, 8, 16 or 32 (as set by CFS) Power frequency range selection Thepowerfrequencyrangeselectedheredefinesthemaximum limit of the power frequency. The operating power frequency is controlled by the 12-bit Power Frequency Select (PFS) word in the control register but may not exceed the value set here. k 512 x n fCARR = MICROPROCESSOR BUS TIMING Intel Mode (Fig. 4 and Table 1) The address is latched by the falling edge of ALE. Data is written from the bus into the SA828 on the rising edge of WR. RD is not used in this mode because the registers in the SA828 are write only. However, this pin must be connected to RD (or tied high) to enable the SA828 to select the correct interface format. Motorola Mode (Fig. 5 and Table 2) The address is latched on the falling edge of the AS line. Data is written from the bus into the SA828 (only when R/W is low) on the falling edge of DS (providing CS is low). CONTROLLING THE SA828 The SA828 is controlled by loading data into two 24-bit registers via the microprocessor interface. These registers are the initialisation register and the control register. The initialisation register would normally be loaded before motor operation (i.e., prior to the PWM outputs being activated) and sets up the basic operating parameters associated with the motor and inverter. This data would not normally be updated during motor operation. The control register is used to control the PWM outputs (and hence the motor) during operation e.g., stop/start, speed, forward/reverse etc. and would normally be loaded and changed only after the initialisation register has been loaded. As the MOTEL bus interface is restricted to an 8-bit wide format, data to be loaded into either of the 24-bit registers is first written to three 8-bit temporary registers R0, R1 and R2 before being transferred to the desired 24-bit register. The data is accepted (and acted upon) only when transferred to one of the 24-bit registers. Transfer of data from the temporary registers to either the initialisation register or the control register is achieved by a write instruction to a dummy register. Writing to dummy register R3 results in data transfer from R0, R1 and R2 to the control register, while writing to dummy register R4 transfers data from R0, R1 and R2 to the initialisation register. It does not matter what data is written to the dummy registers R3 and R4 as they are not real registers. It is merely the write instruction to either of these registers which is acted upon in order to load the initialisation and control registers. |
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