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5.5 Ultra DMA Feature Set
C141-E050-02EN 5-77
5.5 Ultra DMA Feature Set
5.5.1 Overview
Ultra DMA is a data transfer protocol used with the READ DMA and WRITE
DMA commands. When this protocol is enabled it shall be used instead of the
Multiword DMA protocol when these commands are issued by the host. This
protocol applies to the Ultra DMA data burst only. When this protocol is used
there are no changes to other elements of the ATA protocol (e.g.: Command
Block Register access).
Several signal lines are redefined to provide new functions during an Ultra DMA
burst. These lines assume these definitions when 1) an Ultra DMA Mode is
selected, and 2) a host issues a READ DMA or a WRITE DMA, command
requiring data transfer, and 3) the host asserts DMACK-. These signal lines revert
back to the definitions used for non-Ultra DMA transfers upon the negation of
DMACK- by the host at the termination of an Ultra DMA burst. All of the
control signals are unidirectional. DMARQ and DMACK- retain their standard
definitions.
With the Ultra DMA protocol, the control signal (STROBE) that latches data from
DD (15:0) is generated by the same agent (either host or device) that drives the
data onto the bus. Ownership of DD (15:0) and this data strobe signal are given
either to the device during an Ultra DMA data in burst or to the host for an Ultra
DMA data out burst.
During an Ultra DMA burst a sender shall always drive data onto the bus, and
after a sufficient time to allow for propagation delay, cable settling, and setup
time, the sender shall generate a STROBE edge to latch the data. Both edges of
STROBE are used for data transfers so that the frequency of STROBE is limited
to the same frequency as the data. The highest fundamental frequency on the
cable shall be 16.67 million transitions per second or 8.33 MHz (the same as the
maximum frequency for PIO Mode 4 and DMA Mode 2).
Words in the IDENTIFY DEVICE data indicate support of the Ultra DMA feature
and the Ultra DMA Modes the device is capable of supporting. The Set transfer
mode subcommand in the SET FEATURES command shall be used by a host to
select the Ultra DMA Mode at which the system operates. The Ultra DMA Mode
selected by a host shall be less than or equal to the fastest mode of which the
device is capable. Only the Ultra DMA Mode shall be selected at any given time.
All timing requirements for a selected Ultra DMA Mode shall be satisfied.
Devices supporting Ultra DMA Mode 2 shall also support Ultra DMA Modes 0
and 1. Devices supporting Ultra DMA Mode 1 shall also support Ultra DMA
Mode 0.
An Ultra DMA capable device shall retain its previously selected Ultra DMA
Mode after executing a Software reset sequence. An Ultra DMA capable device
shall clear any previously selected Ultra DMA Mode and revert to its default non-
Ultra DMA Modes after executing a Power on or hardware reset.