
Architecture Overview
3-4
GMS30C7201 Data Sheet
3.2
SDRAM controller
3.2.1 Overview
The SDRAM controller is a key part of the GMS30C7201 architecture. The SDRAM
controller has two data ports
—
one for video DMA and one for the main ASB
—
and
interfaces to a single 16-bit wide SDRAM. One to four 16, 64 or 128Mbit x16-bit devices
are supported, giving a memory size ranging from 2 to 64Mbytes.
3.2.2 Arbitration
The main ASB and video DMA buses are independent, and operate concurrently. The
SDRAM controller contains the arbitration logic, for selecting between the two buses,
and between the LCD and VGA on the video bus. The video bus is always higher
priority than the main bus. The SDRAM controller uses a modified round-robin
arbitration between the LCD and the VGA, but allows the highest priority device on the
video bus to be programmed as either the LCD or the VGA.
The video interface consists of separate address and request inputs to the SDRAM
controller, and shared data but individual acknowledge outputs to the VGA and the
LCD. The video access burst size is fixed to 16 words. The address is non-incrementing
for words within a burst (as the SDRAM controller only makes use of the first address
for each burst request).
The arbitration scheme is modified round-robin. When the bus is idle, prioritization is
fixed, with whichever LCD or VGA is programmed as highest priority getting data next,
if they request data at the same time. When the bus is busy, the prioritization becomes
round-robin, so if the higher priority device wants two bursts one immediately after the
other, but the other device also requested a burst at the same time as the higher priority
device, (or after, but before the first burst to the other device completed), then the lower
priority device would get the second burst, and the higher priority device the third burst.
In use, the highest bandwidth video device (VGA or LCD) should be programmed as
highest priority in the SDRAM controller. If both devices are equal priority, and use the
same bandwidth, an arbitrary decision can be made. If only one video device is being
used (for example, only LCD is being used, as will most often be the case), then that
device should have the highest priority.