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Power-on Reset

Serial Interface

DAC8830-EP

DAC8831-EP

SGLS334C – AUGUST 2006 – REVISED APRIL 2007

THEORY OF OPERATION (continued)

Both devices have a power-on reset function to ensure the output is at a known state upon power up. In the

DAC8830 and DAC8831, on power up, the DAC latch and input registers contain all 0s until new data is loaded

However, the serial register of the DAC8830 and DAC8831 is not cleared on power up, so its contents are

undefined. When loading data initially to the device, 16 bits or more should be loaded to prevent erroneous data

appearing on the output. If more than 16 bits are loaded, the last 16 are kept; if less than 16 are loaded, bits will

remain from the previous word. If the device must be interfaced with data shorter than 16 bits, the data should

be padded with 0s at the LSBs.

The digital interface is standard 3-wire connection compatible with SPI, QSPI, Microwire, and Texas Instruments

DSP interfaces, which can operate at speeds up to 50 Mbps. The data transfer is framed by CS, the chip select

signal. The DAC works as a bus slave. The bus master generates the synchronize clock, SCLK, and initiates the

transmission. When CS is high, the DAC is not accessed, and the clock SCLK and serial input data SDI are

ignored. The bus master accesses the DAC by driving pin CS low. Immediately following the high-to-low

transition of CS, the serial input data on pin SDI is shifted out from the bus master synchronously on the falling

edge of SCLK, and latched on the rising edge of SCLK into the input shift register, MSB first. The low-to-high

transition of CS transfers the contents of the input shift register to the input register. All data registers are 16 bit.

It takes 16 clocks of SCLK to transfer one data word to the parts. To complete a whole data word, CS must go

high immediately after 16 SCLKs are clocked in. If more than 16 SCLKs are applied during the low state of CS,

the last 16 bits are transferred to the input register on the rising edge of CS. However, if CS is not kept low

during the entire 16 SCLK cycles, data is corrupted. In this case, reload the DAC latch with a new 16-bit word.

In the DAC8830, the contents of the input register are transferred into the DAC latch immediately when the input

register is loaded, and the DAC output is updated at the same time.

The DAC8831 has an LDAC pin allowing the DAC latch to be updated asynchronously by bringing LDAC low

after CS goes high. In this case, LDAC must be maintained high while CS is low. If LDAC is tied permanently

low, the DAC latch is updated immediately after the input register is loaded (caused by the low-to-high transition

of CS).

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