Jameco Electronics Rabbit 3000 - Manual (Page 280)

User’s Manual 271
an asynchronous signal suitable for the PC. Since the target controls the clock for both
send and receive, the data transmission proceeds at a rate controlled by the target board
under development.
This scheme does not allow for an interrupt, and it is not desirable to use up an external
interrupt for this purpose. The serial port may be used, if desired, During program load
because there is no conflict with the user’s program at compile load time. However, the
user’s program will conflict during debugging. The nature of the transmissions during
debugging is such that the user program starts at a break point or otherwise wants to get
the attention of the PC. The other type of message is when the PC wants to read or write
target memory while the target is running.
The target toggling the clock can simply send a clocked serial message to get the attention
of the PC. The intermediate communications board can accept these unsolicited messages
using its clocked serial port. To prevent overrunning the receiver, the target can wait for a
handshake signal on one of the SMODE lines or there can be suitable pre-arranged delays.
If the PC wants attention from the target it can set a line to request attention (SMODE).
The target will detect this line in the periodic interrupt routine and handle the complete
message in the periodic interrupt routine. This may slow down target execution, but the
interrupts will be enabled on the target while the message is read. The intermediate board
could split long messages into a series of shorter messages if this is a problem.
A.3 Suggested Rabbit Crystal Frequencies
Table A-1 provides a list of suggested Rabbit operating frequencies. The numbers in
Table A-1 are based on the following assumptions:
• spectrum spreader set to normal,
• doubler in use (52/48 duty cycle), and
• a combined 6 ns for clock to address and data setup times.
The crystal can be half the operating frequency if the clock doubler is used up to 27 MHz.
Beyond this operating clock speed, it is necessary to use an X1 crystal or an external oscil-
lator because asymmetry in the waveform generated by the oscillator becomes a variation
in the clock speed if the clock speed is doubled.