Rabbit RabbitCore RCM4000 User Manual - page 91

User’s Manual

85

A.3  I/O Buffer Sourcing and Sinking Limit

Unless otherwise specified, the Rabbit I/O buffers are capable of sourcing and sinking 

8 mA of current per pin at full AC switching speed. Full AC switching assumes a 

29.4 MHz CPU clock with the clock doubler enabled and capacitive loading on address 

and data lines of less than 70 pF per pin. The absolute maximum operating voltage on all 

I/O is 3.6 V.

A.4  Bus Loading

You must pay careful attention to bus loading when designing an interface to the 

RCM4000. This section provides bus loading information for external devices.

Table A-5 lists the capacitance for the various RCM4000 I/O ports.

Table A-6 lists the external capacitive bus loading for the various RCM4000 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-6.

Table A-7 lists the loadings for the A/D converter inputs.

Table A-5.  Capacitance of Rabbit 4000 I/O Ports

I/O Ports

Input 

Capacitance

(pF)

Output 

Capacitance

(pF)

Parallel Ports A to E

12

14

Table A-6.  External Capacitive Bus Loading -40°C to +85°C

Output Port

Clock Speed 

(MHz)

Maximum External 

Capacitive Loading (pF)

All I/O lines with clock 

doubler enabled

58.98

100

Table A-7.  A/D Converter Inputs

Parameter

Value

Input Capacitance

4–9.7 pF

Input Impedance

Common-Mode 6 M

Ω

Differential 7 M

Ω