CONNECTION WITH EXTERNAL DEVICES
7902 Group User’s Manual
3-12
3.2 Chip select wait controller
s Area CS0 bus cycle select bit 0 (bits 0, 1)
The combination of this bit and the area CS0 bus cycle select bit 1 (bit 3 at address 8116) selects
the bus cycle at access to area CS0. (Refer to section “3.2.2 External bus operations.”)
s External data bus width select bit (bit 2)
Reading this bit informs the input level at pin BYTE. The external data bus width at access to area
CS0 is decided by the input level at pin BYTE. (When BYTE = Vss level, 16-bit width; when BYTE
= Vcc level, 8-bit width.)
s RDY control bit (bit 3)
This bit decides whether the RDY control is valid or not at access to area CS0.
While the RDY input select bit (bit 2 at address 5F16) = “1,” this bit is valid. (Refer to section “3.3
Ready function.”)
s Burst ROM access select bit (bit 5)
When ROM, etc., supporting burst access, is allocated to area CS0, the burst access for the
maximum of 8 bytes becomes available if this bit is set to “1.” The burst ROM access is valid only
when the external data bus width = 16 bits with instructions prefetched. When the external data
bus width = 8 bits or when data is read or written, “normal access” is specified regardless of this
bit’s contents. (Refer to section “3.2.2 External bus operations.”)
s Recovery cycle insert select bit (bit 6)
This bit decides whether recovery cycles are inserted or not at access to area CS0. Setting this
bit to “1” inserts such recovery cycles as 1 or 2 cycles of
φ1 after the bus cycle for access to area
CS0. The number of recovery cycles to be inserted is selected by the recovery-cycle-insert number
select bit (bit 6 at address 5F16). Insertion of recovery cycles allows devices with longer output
disable time at read to be connected without using bus buffers.
Since addresses are maintained throughout recovery cycles, devices requiring longer address hold
time can easily be connected; on the other hand, by inserting 2 recovery cycles to extend the data
hold time at write by 1 cycle of
φ1, devices requiring longer data hold time can also be connected.
(Refer to section “3.2.2 External bus operations.”)
s CS0 output select bit (bit 7)
Setting this bit to “1” outputs a chip select signal at access to area CS0.
Even though this bit has been cleared to “0” in order to disable CS0 output, setting for each
function of area CS0 (See Table 3.2.1.) is valid if the area CS0 block size select bits (bits 2 to 0
at address 8116) are not “0002” (in other words, area CS0 is invalid.)
Moreover, even when area CS0 is invalid, setting this bit to “1” validates pin CS0. (“H” level is
output.)