
Architecture Overview
3-3
GMS30C7201 Data Sheet
3.1.2 ASB
The ASB is designed to allow the ARM to have continuous access to both the ROM/
PCMCIA interface and the SDRAM. The SDRAM controller straddles both the ASB and
the video DMA bus so the LCD and VGA can access the SDRAM controller
simultaneously with activity on the ASB. This means that the ARM or Piccolo can read
code from ROM, or access a peripheral, without being interrupted by video DMA.
The GMS30C7201 uses a modified arbiter to control mastership on the main ASB bus.
The arbiter only arbitrates on quad-word boundaries, or when the bus is idle. This is to
get the best performance with the ARM720T, which uses a quad-word cache line, and
also to get the best performance from the SDRAM, which uses a burst size of eight
halfwords per access. By arbitrating only when the bus is idle or on quad-word
boundaries (A[3:2] = 11), it ensures that cache line fills are not broken up, hence
SDRAM bursts are not broken up.
Video ASB arbitration is controlled by the SDRAM controller. This is explained in
3.2.2
Arbitration
on page 3-4.
3.1.3 Video bus
The video bus hosts the LCD controller and the VGA controller DMA. The video bus
consists of separate address inputs, a request / acknowledge to / from the SDRAM
controller, for each of the LCD and VGA blocks, and a shared data bus. The LCD and
VGA registers are programmed from the fast APB. The SDRAM controller arbitrates
between ASB, VGA and LCD access requests. Video always has higher priority than
ASB access requests. The split ASB/video bus architecture of the SDRAM controller
allows slow device accesses
—
such as access to a PC-Card that asserts a
WAIT
signal
for several microseconds
—
without blocking video DMA.
3.1.4 APBs
There are two APB buses. These are the
fast
and
slow
APB buses. The fast APB bus
operates at the speed of the ASB (30 MHz), and hosts the Fast and Medium speed
infra-red interface, the USB interface, the sound output interface, and the LCD and VGA
registers. These are the high performance peripherals, which are generally DMA
targets.
The slow APB peripherals generally operate at the UART crystal clock frequency of
3.6864MHz, though register access via the APB is at ASB speed. The slow APB
peripherals do not support DMA transfers. This arrangement of operating most of the
peripherals from a slower clock, and reducing the load on the faster bus, results in
significantly reduced power consumption. Both APB buses connect to the main ASB
bus via specially modified bridges.The slow APB bridge takes care of all re-
synchronization, handing over data and control signals between the ASB and UART
clock domains in a safe and reliable manner.
The fast APB bridge is modified from the normal AMBA bridge, to allow DMA access
to fast APB peripherals. Additional signals from the DMA controller to the APB bridge
request, select and acknowledge DMA transfers to and from DMA-aware peripherals.