Overview of the Memory and I/O Spaces
4-4
Table 4–1. Pins for Interfacing With External Memory and I/O Spaces (Continued)
Description
Pin(s)
Read/write
signals
R/W
Read/write pin. This pin indicates the direction of transfer between the
’C2xx and external program, data, or I/O space.
RD
Read select pin. The ’C2xx asserts RD to request a read from external
program, data, or I/O space.
WE
Write enable pin. The ’C2xx asserts WE to request a write to external pro-
gram, data, or I/O space.
Request/control
signals
BOOT
Boot load pin. This pin is only on devices that have the on-chip boot load-
er. If BOOT is low during a hardware reset, the ’C2xx transfers code from
EPROM in global data memory to RAM in external program memory.
MP/MC
Microprocessor/microcomputer pin. This pin is only on devices with on-
chip non-volatile program memory. The level on this pin is tested at reset.
If MP/MC is high, the device is in microprocessor mode (the reset vector
is fetched from external memory). If MP/MC is low, the device is in micro-
computer mode (the reset vector is fetched from on-chip memory).
RAMEN
Single-access RAM enable pin. On ’C2xx devices with on-chip single-ac-
cess RAM, when this pin is high, the RAM is enabled; when this pin is low,
the RAM is disabled.
READY
External device ready pin (for generating wait states externally). When
this pin is driven low, the ’C2xx waits one CPU cycle and then tests
READY again. After READY is driven low, the ’C2xx does not continue
processing until READY is driven high. If READY is not used, it should
be kept high. On the ’C203, at boot time, this pin must be high.
HOLD
HOLD operation request pin. An external device can request control of
the external buses by asserting HOLD. After the ’C2xx (along with proper
software logic) asserts HOLDA, the external device controls the buses
until it deasserts HOLD.
HOLDA
HOLD acknowledge pin. The ’C2xx (with assistance from proper pro-
gram code) asserts HOLDA to acknowledge that HOLD has been as-
serted and places its external buses in high impedance.
