ADLINK PCI-7300A - Manual (Page 50)

38 Operation Theory
4.7 Clocking Mode
The data input to or output from the FIFO is operated in a specific
rate. The specific sampling rate or the pacer rate can be program-
mable by software, by external clock, or by easy handshaking pro-
tocol.
Four clocking modes are provided in the cPCI/PCI-7300A to sam-
ple input data to the FIFO or output date from FIFO to the external
devices. They are:
1. Internal Clock: Three sources are available to activate
both digital input and digital output. They are 20MHz,
10MHz, and programmable timer 82C54. There are
three counters in 82C54, counter 0 is used to generate
sampling clock for digital input, counter 1 is used timer
pacer for digital output, and counter 2 is used for inter-
rupt function. The configuration is illustrated as follows.
Figure 4-6: Timer configuration
2. External Clock: This mode is only applied for digital
input. The digital inputs are handled by the external clock
strobe (DI-REQ). The DI-ACK signal reflects the almost
full status of the input FIFO. The DI-ACK is asserted
when input FIFO is not almost full, which means the
external device can input data. If the input FIFO is
almost full, the DI-ACK is de-asserted, then the external
device should pause data transfer and wait for the asser-