PSD813F2, PSD833F2, PSD834F2, PSD853F2, PSD854F2
80C251
The Intel 80C251 MCU features a user-config-
urable bus interface with four possible bus config-
urations, as shown in Table 18., page 48.
The first configuration is 80C31-compatible, and
the bus interface to the PSD is identical to that
shown in Figure 21., page 46. The second and
third configurations have the same bus connection
as shown in Figure 22. There is only one Read
Strobe (PSEN) connected to CNTL1 on the PSD.
The A16 connection to PA0 allows for a larger ad-
dress input to the PSD. The fourth configuration is
shown in Figure 23., page 48. Read Strobe (RD) is
connected to CNTL1 and Program Select Enable
(PSEN) is connected to CNTL2.
The 80C251 has two major operating modes:
Page mode and Non-page mode. In Non-page
mode, the data is multiplexed with the lower ad-
dress byte, and Address Strobe (ALE/AS, PD0) is
active in every bus cycle. In Page mode, data (D7-
D0) is multiplexed with address (A15-A8). In a bus
cycle where there is a Page hit, Address Strobe
(ALE/AS, PD0) is not active and only addresses
(A7-A0) are changing. The PSD supports both
modes. In Page Mode, the PSD bus timing is iden-
tical to Non-Page Mode except the address hold
time and setup time with respect to Address
Strobe (ALE/AS, PD0) is not required. The PSD
access time is measured from address (A7-A0)
valid to data in valid.
Figure 22. Interfacing the PSD with the 80C251, with One READ Input
RESET
80C251SB
2
3
4
5
6
P1.0
P1.1
P1.2
P1.3
7
8
9
P1.4
P1.5
P1.6
P1.7
21 X1
20 X2
11
13
P3.0/RXD
P3.1/TXD
14
15 P3.2/INT0
16
17
P3.3/INT1
P3.4/T0
P3.5/T1
10 RST
35
EA
P0.0 43
P0.1
P0.2
42
41
P0.3 40
P0.4 39
P0.5 38
P0.6 37
P0.7 36
P2.0 24
P2.1 25
P2.2 26
P2.3 27
P2.4 28
P2.5 29
P2.6 30
P2.7 31
ALE 33
32
PSEN
18
WR
19
RD/A16
A0
A1
A2
A3
A4
A5
A6
A7
AD8
AD9
AD10
AD11
AD12
AD13
AD14
AD15
ALE
RD
WR
A16
RESET
RESET
Note: 1. The A16 and A17 connections are optional.
2. In non-Page-Mode, AD7-AD0 connects to ADIO7-ADIO0.
A0
A1
A2
A3
A4
A5
A6
A7
AD8
AD9
AD10
AD11
AD12
AD13
AD14
AD15
PSD
30
31
32
ADIO0
ADIO1
ADIO2
33 ADIO3
34 ADIO4
35 ADIO5
36 ADIO6
37 ADIO7
39
40
ADIO8
ADIO9
41 ADIO10
42
43
ADIO11
ADIO12
44 ADIO13
45 ADIO14
46 ADIO15
47 CNTL0 (WR)
50 CNTL1(RD)
49 CNTL 2(PSEN)
10
9
PD0- ALE
PD1
8 PD2
48
RESET
29
PA0 28
PA1
PA2 27
PA3
PA4
PA5
25
24
23
PA6
PA7
22
21
A161
A171
7
PB0 6
PB1 5
PB2
PB3
PB4
4
3
PB5 2
PB6 52
PB7 51
PC0
PC1
20
19
PC2 18
PC3
PC4
17
14
13
PC5
PC6
PC7
12
11
AI02881C
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