Galil DMC-1800, DMC-1700 FIFO Control Register at N+4, Half Full Flag, Enabling and Reading IRQ’s

FIFO Control Register at N+4

Status Bit	Read/Write	Meaning
7	Read Only	If 1, Secondary FIFO empty
6	Read/Write	IRQ enable: Write 1 to enable IRQ
		Write 0 to disable IRQ
		Read 1 = IRQ enabled
5	Read/Write	IRQ status: Write 1 to clear IRQ
		Read 1 = IRQ pending
4	Read/Write	Freeze Status of Secondary FIFO:
		Write 1 to freeze 2nd FIFO
		Write 0 to clear freeze of 2nd FIFO
		Read 1 = 2nd FIFO frozen
3	Read Only	If 1, Secondary FIFO is busy updating
2	Read Only	If 1, DMC to PC Buffer empty, No data to be read
1	Read Only	If 0, PC to DMC buffer not half full. Can write at least 255 bytes.
		If 1, buffer is more than half full.
0	Read Only	If 1, PC to DMC Buffer full, Do not write data

Half Full Flag

The Half Full flag (Bit 1 of the control register) can be used to increase the speed of writing large blocks of data to the controller. When the half full bit is zero, the write buffer is less than half full. In this case, up to 255 bytes can be written to the controller at address N without checking the buffer full status (bit 0 of the control register).

Reading the Data Record from the Secondary FIFO

To read the data record from the secondary FIFO, first the “freeze” bit (bit 4 of N+4) of the control register must be set, Then wait for the controller to finish updating the data record by monitoring the “busy status bit (bit3 of N+4), when bit

3 is “0” the data record can be read. Since the Secondary FIFO at N+C is 4 bytes wide, data may be read in 1 byte, 2 byte or 4 byte increments. Read the data at N+C until bit 7 of N+4 is 1, signifying that the FIFO is empty. After the data has been read, un-freeze the secondary FIFO by setting bit 4 of N+4 to “0”, which allows the controller to continue to refresh the data record at the defined rate specified by the DR command.

Enabling and Reading IRQ’s

In order to service interrupts from the IRQ line, the IRQ control register (Status Byte) must first be enabled. This is done by setting bit 6 of the control register (N+4) equal to “1”.

When interrupted, a device driver’s interrupt service routine must verify that the interrupt originated from the DMC- 1800 controller. This is done by checking that the IRQ enable and IRQ status bits (bit 5 and 6 of N+4) are high. The Status Byte can then be read by reading the register at N+8. The returned Status Byte indicates what event generated the interrupt (for more information on specific interrupt events, see the EI and UI commands in the Command Reference or the previous section “Controller Event Interrupts…” in this chapter).

Once the Status Byte has been read, the interrupt must be cleared by writing a “1” to bit-5 of N+4. Note: to preserve values of other bits, the interrupt service routine should read N+4, set bit 5, and write this value back to N+4 to clear the interrupt.

72 • Chapter 4 - Software Tools and Communications DMC-1700/1800