User`s manual

User’s Manual 31
A.2 Bus Loading
When designing an interface to the RabbitCore 2000, careful attention must be paid to bus
loading. This section provides bus loading of external devices.
Table A-3 lists the capacitance for the various RabbitCore 2000 I/O ports.
Table A-4 lists the external capacitive bus loading for the various Rabbit 2000 output
ports. Be sure to add the loads for the devices you are using in your custom system and
verify that they do not exceed the values in Table A-4.
Table A-3. Capacitance of Rabbit 2000 I/O Ports
I/O Ports
Input
Capacitance
(pF)
Output
Capacitance
(pF)
Parallel Ports A to E 12 14
Data Lines D0–D7 30 32
Address Lines A0–A12 30 32
Table A-4. External Capacitive Bus Loading -40°C to +85°C
Output Port
Clock Speed
(MHz)
Maximum External
Capacitive Loading (pF)
A[12:1]
D[7:1]
25.8 50
A[12:1]
D[7:1]
18.4
55 for 90 ns flash
100 for 55 ns flash
*
* The RCM2020 operating at 18.4 MHz will typically come with a
flash EPROM whose access time is 55 ns. Because of the volatil-
ity of the memory market, a 90 ns flash EPROM could be used on
the RCM2020.
A0
D0
25.8, 18.4 100
PD[3:0] 25.8, 18.4, 100
PA[7:0]
PB[7,6,1,0]
PC[6,4,2,0]
PD[7:4]
PE[7:0]
25.8, 18.4 90
All data, address, and
I/O lines with clock
doubler disabled
12.9, 9.2 100