Honeywell ISA100 Gen X User Manual page 26

Parameter
Ts
Tb
Tf
Td
Packet Size and buffers
The SPI message definition allows for a 7-bit data length field. This limits the SPI message size
to 127 bytes. However, the actual maximum message size is limited by the size of the SPI Rx
buffer in the Radio board. In the ISA100 Radio board, the SPI Rx, and the Tx buffer size is
only 80 bytes. Hence the Sensor board must not send packets exceeding this buffer size.
Interrupt processing
There are three interrupts on the Radio board that are used in the SPI interface.
Radio SPI interrupts
Sensor Data Ready (SDR)
The Sensor Data Ready interrupt is generated from the Sensor board to indicate that the data
is ready to be sent to the Radio board. The Radio acknowledges this interrupt by reading the
SPI packet from the Sensor.
SPI Rx Data
The Radio uses the SPI Rx data interrupt to receive the data. Each time the Master receives an
Rx data interrupt, it reads the data from the SPI buffer and places a new byte to send in the SPI
Tx buffer. The SPI Tx data interrupt is not required to allow a faster and lower power
interface. As the SPI is buffered twice, this mechanism allows the SPI data to be transferred as
a continuous bit stream.
26
Description
Time from CS active until start of first
Master Byte transfer
Time for a Byte transfer
Time from last bit sent until CS
inactive.
Time from CS inactive until next CS
active.
ISA100 Gen X Radio Module User's Guide
Honeywell
Minimum Maximum
0 -
32768/(2 * -
8) = 0.5
msec
0 -
10 msec -
Release Independent
April 2018