668-0003-C Rabbit Semiconductor, 668-0003-C Datasheet - Page 141

• Manufacturer Part Number: 668-0003-C
• Description: IC CPU RABBIT2000 30MHZ 100PQFP
• Manufacturer: Rabbit Semiconductor

Specifications of 668-0003-C

• Rohs Status: RoHS non-compliant
• Processor Type: Rabbit 2000 8-Bit
• Speed: 30MHz
• Voltage: 2.7V, 3V, 3.3V, 5V
• Mounting Type: Surface Mount
• Package / Case: 100-MQFP, 100-PQFP
• Features: -
• Other names: 316-1004; 668-0003

address of the slave, then the “wake up” mode is turned off so that the remaining charac-
ters in the frame can be read. This scheme reduces the overhead associated with messages
directed to other slaves, but it does not really help with the worst-case load. In most cases,
the worst-case compute load is the governing factor for embedded systems. In addition, it
is quite easy for the interrupt driver to dismiss characters not directed to the system. For
these reasons, the “wake up” mode was not implemented for the Rabbit.
The 9th bit protocols suffer from a major problem that the IBM-PC uarts can support the
9th bit only by using special drivers.
12.7.7 Rabbit-Only Master/Slave Protocol
If only Rabbit microprocessors are connected, the 9th bit low can be set on the address
byte, and the remaining bytes can be transmitted in the normal 8-bit mode. This is more
efficient than other 9th bit protocols because only the first byte requires 11 baud times; the
remaining bytes are transmitted in 10 baud times.
12.7.8 Data Framing/Modbus
Some protocols, for example, Modbus, depend on a gap in the data frame to detect the
beginning of the next frame. The 9th bit protocol is another way to detect the start of a data
frame.
The Modbus protocol requires that data frames begin with a minimum 3.5-character quiet
time. The receiver uses this 3.5-character gap to detect the start of a frame. In order for the
receiving interrupt service routine to detect this gap, it is suggested that dummy characters
be transmitted to help detect the gap. This can be done in the following manner. The trans-
mitter starts transmitting dummy characters when the first character interrupt is received.
Each time there is an interrupt, either receiver data register full or transmitter data register
empty, a dummy character is transmitted if the transmitter data register is empty. Although
the transmitter and receiver operate at approximately the same baud rate, there can be a
difference of up to about 5% between their baud rates. Thus the receiver full and transmit-
ter empty interrupts will become out of phase with each other, assuming that the remote
station transmits without gaps between characters. A counter is zeroed each time a charac-
ter is received, and the counter is incremented each time a character is transmitted. If this
counter holds (n), this indicates that a gap has been detected in the frame; the length of the
gap is (n - 1) to (n) characters. The start of frame could be marked by (n) reaching 3, indi-
cating that the existence of a gap at least two characters long.
Chapter 12 Rabbit Serial Ports
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