BCM1250/BCM1125/BCM1125H
User Manual
10/21/02
B r o a d c o m C o r p o r a t i o n
Page
368
Section 11: Generic/Boot Bus
Document
1250_1125-UM100CB-R
A
CKNOWLEDGEMENT
R
EAD
A
CCESS
Figure 76: Acknowledge Read Access
The start of an acknowledgement access is the same as a fixed cycle, however after IO_CS_L is asserted and
the cs_width interval has passed the IO_RDY line is monitored. The IO_RDY line is treated as an
asynchronous signal, and therefore synchronized internally to the 100 MHz reference clock (if it fails to meet
the rdy_setup time it may not be sampled until the next cycle) so it should be asserted for a minimum of 20 ns.
Once IO_RDY has been asserted (to match the rdy_active control bit) and the rdy_smple delay has passed
the data will be sampled and IO_OE_L deasserted. Zero to three cycles later, set by the oe_to_cs parameter,
IO_CS_L will be deasserted. If the device does not signal that it is ready within the timeout period then the read
will be aborted, the timeout error status set, the io_error_int interrupt will be raised and UNPREDICTABLE data
flagged with a bus error will be returned.
In the case where the peripheral is using the IO_CLK100 and can meet the setup and hold time for IO_RDY
the synchronizer may be bypassed. There will therefore be no delay in the acknowledgement reaching the
generic bus state machine and it will take effect immediately. For a read if the rdy_smpl parameter is set to
zero and the synchronizer is bypassed the data is latched by the same IO_CLK100 edge that the IO_RDY is
setup to and the IO_OE_L will deassert following that clock edge (by the clock-to-out delay).
rdy_smple
ale_to_cs
idle_cycle
ale_width
rdy_setup
cs_width
1 cycle
Address
Data
Address
Data
Non Muxed
Muxed
Parity
rdy_active = 0
clk100
io_ale
io_ad[23:0]
io_ad[31:24]
io_ad[31:0]
io_adp[3:0]
io_cs_l[n]
io_oe_l
io_w_lr
io_rdy
cs_to_oe
oe_to_cs