Maxim Integrated MAXQ Family - Manual (Page 100)

9-22
MAXQ Family User’s Guide
9.5.2.2 Receive Pin Sampling
When IREN = 1 and IRTX = 0, the IR hardware supports the T2H register counting of internal T2L edges just as described for the IR
Transmit mode, but the function of the IRBB bit changes. The IRBB bit is used to store the state of the T2P input pin when a compare
match occurs between the T2H and T2CH registers. Additionally, the CCF[1:0] bits define which edge(s) of the T2 pin should trigger
reloads of the T2H counter to allow some form of synchronization when slightly different transmitter/receiver carrier frequencies and bit
timing exist. The user software would be responsible for reading the IRBB sampled pin states and recreating, based upon the IR encod-
ing format, the actual received data. The IRBB bit can be overwritten, thus the user software is responsible for reading the IRBB bit
between compare matches to avoid loss of captured pin data.
9.5.2.3 IR Decoding (Receive) Example
One possible decoding configuration is shown in Figure 9-13 with the T2CH register configured to produce a match after approximately
1/4 of the subcarrier cycles present during a fixed bit time. Each of the two IRBB samples could be examined to determine one
received bit.
Bit length decoding could use a similar strategy, possibly configuring the T2CH match register to 3/4 the number of subcarrier cycles
present in a 0 bit time. The collected IRBB pattern could simply be inverted to produce the actual input stream.
T2P INPUT
RELOAD (FALLING)
CCF[1:0] = 10b
IRBB
(T2CH MATCH)
1 0 1 0 0 0 1 1 0 1
Figure 9-13. Biphase Decoding Example
00h T2CL T2RL FFh
T2CLK
T2POL[1] T2MD = 1
T2OE[1] = 0
PORT LATCH
(IF PDx.x = 1)
T2PB
PIN
T2L 8-BIT TIMER
T2H 8-BIT COUNTER
T2P PIN
GATE
IREN
IRBB
SAMPLE T2P ON
T2CH MATCH
CCF[1:0]
T2P PIN
1
0
1
0
IREN = 1
IRTX = 0
00h FFh
T2RH
T2RH FFh
T2RH
T2CH T2CH
EDGE
RELOAD
Figure 9-12. IR Receive Pin Sampling
Maxim Integrated