PXD20RM Datasheet, PDF (601/1628 Pages) Freescale Semiconductor, Inc – PXD20 Microcontroller

Chapter 14
DRAMC Priority Manager
14.1 Introduction
The multi-port DRAM controller (DRAMC) has built-in arbiters that use a 4-bit priority signal. The
DRAMC will try to service the request with the highest priority first. As a result of this, the DRAMC itself
has no built-in way to fairly split the DRAM bandwidth over the different masters, taking into account their
requirements.
The task of setting the priorities is done by this block, the priority manager. It will in a dynamic way set
the priorities of the 7 DRAM busses in such a way that bandwidth is divided fairly, and that each master
gets its fair share of the bandwidth.
The priority manager does this in a dynamic way. If a channel is not serviced during some time, it will
increase its priority. If a channel is serviced, it will decrease the priority.
Operation of the priority manager can be configured under software control.
A block diagram of the priority manager is given in Figure 14-1. It accepts the request and ACK
(acknowledge) signals for all 7 DRAM busses, and produces the priority signals for the 7 busses.
ipmx0_prio_in (DCU)
ipmx0_req
ipmx1_req
ipmx2_req
ipmx3_req
ipmx4_req
ipmx0_ack
ipmx1_ack
ipmx2_ack
ipmx3_ack
ipmx4_ack
Priority
Manager
ipmx0_prio[0:3]
ipmx1_prio[0:3]
ipmx2_prio[0:3]
ipmx3_prio[0:3]
ipmx4_prio[0:3]
Slave bus
IPS
interface
Figure 14-1. Priority manager block diagram
The priority manager uses an ACK-based schema, meaning that the priority is dependent on how many
times for the last N requests accepted by the DRAMC, the current own bus won the request. The algorithm
is given in Figure 14-2. A running average counter keeps track of how many ACKs on the memory busses
out of the last N ACKs have been for the own bus, this number is then put in a look-up table that is
configurable by writing some config registers, and the output of the look-up table then is the priority for
the next request on this bus.
Freescale Semiconductor
PXD20 Microcontroller Reference Manual, Rev. 1
Preliminary—Subject to Change Without Notice
14-1