82077AA Datasheet, PDF (48/62 Pages) Intel Corporation – CHMOS SINGLE-CHIP FLOPPY DISK CONTROLLER

82077AA
The PC AT disk controller provides direct control of
the drive selects and motors via the Digital Output
Register (DOR) As a result drive selects on the
8272A are not utilized This places drive selection
and motor speed-up control responsibility with the
software The DOR is also used to perform a soft-
ware reset of the disk controller and tristate the
DRQ2 and IRQ6 output signals on the PC bus
The design of the disk controller also requires ad-
dress decode logic for the disk controller and regis-
ter set buffering for both the disk interface and PC
bus support for write precompensation and monitor-
ing of the disk change signal via a separate read
only register (DIR) An I O address map of the com-
plete register set for the PC AT floppy disk control-
ler is shown in Table 9-2
Table 9-2 I O Address Map for the PC AT
IO
Address
Access
Type
Description
3F0H
3F1H
3F2H
3F3H
3F4H
3F5H
3F6H
3F7H
3F7H
Write
Read
Read Write
Write
Read
Unused
Unused
Digital Output Register
Unused
Main Status Register
Data Register
Unused
Data Rate Select Register
Digital Input Register
9 1 2 82077AA PC AT SOLUTION
The 82077AA integrates the entire PC AT controller
design with the exception of the address decode on
a single chip The schematic for this solution is
shown in Figure 9-2 The chip select for the
82077AA is generated by a 16L8 PAL that is pro-
grammed to decode addresses 03F0H thru 03F7H
when AEN (Address Enable) is low The program-
ming equation for the PAL is shown in a ABEL file
format in Figure 9-3 An alternative address decode
solution could be provided by using a 74LS133 13
input NAND gate and 74LS04 inverter to decode
A3 – A14 and AEN Although the PC AT allows for a
64K I O address space decoding down to a 32K
I O address space is sufficient with the existing
base of add-in cards
A direct connection between the disk interface and
the 82077AA is provided by on-chip output buffers
with a 40 mA sink capability Open collector outputs
from the disk drive are terminated at the disk con-
troller with a 150X resistor pack The 82077AA disk
interface inputs contain a schmitt trigger input struc-
ture for higher noise immunity The host interface is
a similar direct connection with 12 mA sink capabili-
ties on DB0 – DB7 INT and DRQ
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