
./01
User
’
s Manual
9-11
1999-08
Read or Write Chip Select
signals remain active only as long as the associated control
signal (RD or WR) is active. This also includes the programmable read/write delay. Read
chip select is only activated for read cycles, write chip select is only activated for write
cycles, read/write chip select is activated for both read and write cycles (write cycles are
assumed, if any of the signals WRH or WRL gets active). These modes save external
glue logic, when accessing external devices like latches or drivers that only provide a
single enable input.
Address Chip Select
signals remain active during the complete bus cycle. For address
chip select signals two generation modes can be selected via bit CSCFG in register
SYSCON:
- A
latched
address chip select signal (CSCFG=
’
0
’
) becomes active with the falling edge
of ALE and becomes inactive at the beginning of an external bus cycle that accesses a
different address window. No spikes will be generated on the chip select lines and no
changes occur as long as locations within the same address window or within internal
memory (excluding X-Peripherals and XRAM) are accessed.
- An
early
address chip select signal (CSCFG=
’
1
’
) becomes active together with the
address and BHE (if enabled) and remains active until the end of the current bus cycle.
Early address chip select signals are not latched internally and may toggle intermediately
while the address is changing.
Note: CS0 provides a latched address chip select directly after reset (except for single
chip mode) when the first instruction is fetched.
Internal pullup devices hold all CS lines high during reset. After the end of a reset
sequence the pullup devices are switched off and the pin drivers control the pin levels
on the selected CS lines. Not selected CS lines will enter the high-impedance state and
are available for general purpose IO.
Segment Address versus Chip Select
The external bus interface of the C161PI supports many configurations for the external
memory. By increasing the number of segment address lines the C161PI can address a
linear address space of 256 KByte, 1 MByte or 8 MByte. This allows to implement a
large sequential memory area, and also allows to access a great number of external
devices, using an external decoder. By increasing the number of CS lines the C161PI
can access memory banks or peripherals without external glue logic. These two features
may be combined to optimize the overall system performance.
Note: Bit SGTDIS of register SYSCON defines, if the CSP register is saved during
interrupt entry (segmentation active) or not (segmentation disabled).