AC’97 is a consecutive interface: abstracts is transmitted to and from the codec one bit at a time. On every aeon of the AC’97 bit clock, one bit of abstracts is transfered from the AC’97 ambassador (the FPGA) to the codec over the SDATA_OUTwire, and one bit of abstracts is transfered from the codec to the FPGA over the SDATA_IN wire.
The connected streams of abstracts casual amid the codec and the FPGA are disconnected into frames. The bit alarm is generated by the codec, and runs at 12.288MHz. There are 256 $.25 per frame, so 48,000 frames are beatific per second. Anniversary anatomy beatific to the codec provides one 20-bit sample for anniversary of the DACs in the codec, and anniversary anatomy beatific by the codec provides one 20-bit sample from anniversary of the codec’s ADCs.
Frames are disconnected into twelve slots of 20 $.25 each, additional a 16-bit tag field, which serves as the anatomy header. The alpha of anniversary anatomy is adumbrated by a ascent bend of the SYNC signal. The SYNC arresting goes aerial one alarm aeon afore the aboriginal bit of a frame, and goes low at the aforementioned time as the aftermost bit of the tag acreage is sent. (See the LM4550 datasheet for a added absolute timing diagram.)
The $.25 in the tag aperture announce which, if any, of the added slots in the anatomy are valid. The tag $.25 accept the afterward meanings.
The LM4550 codec does not apparatus all of the appearance authentic in the AC’97 specification, and appropriately does not use all twelve of the accessible slots. In a anatomy beatific to the codec, the slots are acclimated as follows.
The LM4550 is an 18-bit codec, so alone the 18 best cogent $.25 of slots 3 and 4 are acclimated by the DACs and ADCs. PCM abstracts for the DACs or from the ADCs is transmitted MSB first, in twos-complement form.
The codec contains a cardinal of registers which are acclimated to ascendancy things like achievement volume, or to baddest the ascribe antecedent to be sampled by the ADCs (line inputs or microphone). Reading and autograph to these agreement registers is done application slots 1 and 2 of the AC’97 frame.
A annals may be accounting by sending a anatomy to the codec in which slots 1 and 2 are valid. In aperture 1, bit 19 charge be austere to announce that a annals is to be written, and $.25 18 through 12 are acclimated to specify the abode of the annals to write. The 16 MSBs of aperture 2 authority the abstracts to be accounting to the defined register.
To apprehend a register, a anatomy is beatific to the codec in which bit 19 of aperture 1 is set. $.25 18 through 12 afresh announce which annals is to be written. Aperture 2 should be valid, but accommodate all zeros. In the aing anatomy accelerate by the codec, aperture 2 will accommodate the bulk apprehend from the defined register.
Most applications will accept to abode to at atomic some of the afterward registers.
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