TC850
6.3.6
CONTINUOUS/DEMAND MODE
INPUT (CONT/DEMAND)
6.3
Pin Description (Digital)
6.3.1
CHIP SELECT AND CHIP ENABLE
(CS AND CE)
This input controls the TC850 operating mode. When
CONT/DEMAND is HIGH, the TC850 performs conver-
sions continuously. In continuous mode, data must be
read in the prescribed sequence shown in Table 6-1.
Also, all three data bytes must be read within 443-1/2
internal clock cycles after the BUSY output goes low.
After 443-1/2 clock cycles data will be lost.
The CS and CE inputs permit easy interfacing to a vari-
ety of digital bus systems. CE is active LOW while CS
is active HIGH. These inputs are logically ANDed
internally and are used to enable the RD and WR
inputs.
When CONT/DEMAND is LOW, the TC850 begins a
conversion each time CS and CE are active and WR is
being pulsed LOW. The conversion is complete and
data can be read after the falling edge of the BUSY out-
put. In demand mode, data can be read in any
sequence and remains valid until WR is again pulsed
LOW.
6.3.2
WRITE ENABLE INPUT (WR)
The write input is used to initiate a conversion when the
TC850 is in demand mode. CS and CE must be active
for the WR input to be recognized. The status of the
data bus is meaningless during the WR pulse, because
no data is actually written into the TC850.
6.3.7
BUSY OUTPUT (BUSY)
6.3.3
READ ENABLE INPUT (RD)
The BUSY output is used to convey an end-of-conver-
sion to external logic. BUSY goes HIGH at the begin-
ning of the de-integrate phase and goes LOW at the
end of the conversion cycle. Data is valid on the falling
edge of BUSY. The output-high period is fixed at 836
clock periods, regardless of the analog input value.
BUSY is active during continuous and demand mode
operation.
The read input, combined with CS and CE, enable the
3-state data bus outputs. Also, in continuous mode, the
rising edge of the RD input activates an internal byte
counter to sequentially read the three data bytes.
6.3.4
LOW/HIGH BYTE SELECT (L/H)
The L/H input determines whether the low (least signif-
icant) byte or high (most significant) byte of data is
placed on the 3-state data bus. This input is meaningful
only when the TC850 is in the demand mode. In the
continuous mode, data must be read in three
predetermined bytes, so the L/H input is ignored.
This output can also be used to generate an end-of-
conversion
interrupt
in
µP-based
systems.
Noninterrupt-driven systems can poll BUSY to deter-
mine when data is valid.
6.3.5
OVERRANGE/POLARITY BIT
SELECT (OVR/POL)
The TC850 provides 15 bits of resolution, plus polarity
and overrange bits. Thus, 17 bits of information must be
transferred on an 8-bit data bus. To accomplish this, the
overrange and polarity bits are multiplexed onto data bit
DB7 of the most significant byte. When OVR/POL is
HIGH, DB7 of the high byte contains the overrange sta-
tus (HIGH = analog input overrange, LOW = input within
full scale). When OVR/POL is LOW, DB7 is HIGH for
positive analog input polarity and LOW for negative
polarity. The OVR/POL input is meaningful only when
CS, CE and RD are active, and L/H is LOW (i.e., the
most significant byte is selected). OVR/POL is ignored
when the TC850 is in continuous mode.
DS21479B-page 14
2002 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
