Preliminary
Gas Sensor Analog Front End Datasheet GasSensorAFE V 1.10
001-81375 Rev. *A
GasSensorAFE
Copyright © 2012-2013 Cypress Semiconductor Corporation. All Rights Reserved.
This datasheet contains Preliminary information.
PSoC® Blocks
Resources
Digital
Analog CT
API Memory (Bytes)
Analog SC
Flash
RAM
Pins (per
External I/O)
Supported Devices: CY8C29466, CY8C29566, CY8C29666, CY8C29866, CY8C28243, CY8C28433,
CY8C28445, CY8C28452, CY8C28533, CY8C28545, CY8C28623, CY8C28643, CY8C28645, CY8C27143,
CY8C27243, CY8C27443, CY8C27543, CY8C27643, CY8C24094, CY8C24794, CY8C24894, CY8C24994,
CY8C24123A, CY8C24223A, CY8C24423A
0
2
1
427
0
4
Features and Overview
Implements a three-lead electrochemical sensor with current output.
Contains a trans-impedance amplifier (TIA) for current-to-voltage conversion.
Maintains bias conditions needed by the cell
Can be used to build a single-chip gas sensing solution in combination with appropriate ADC, SLCD,
and CapSense User Modules
The Gas Sensor Analog Front End (AFE) User Module contains a bias circuit to maintain the sensor at a
specific state and a TIA (current-to-voltage converter) to translate the sensor current, which is proportional
to gas concentration, into a voltage that can be digitized.
Cypress Semiconductor Corporation
Document Number: 001-81375 Rev. *A
•
198 Champion Court
•
San Jose, CA 95134-1709
•
408-943-2600
Revised May 15, 2013
GasSensorAFE
Figure 1.
GasSensor Hardware Block Diagram (PSoC 1)
Figure 2.
Gas Sensor Circuit (PSoC 1)
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GasSensorAFE
Functional Description
The Gas Sensor AFE User Module is made up of two main sections as shown in Figure 1 and Figure 2:
A control amplifier
A trans-impedance amplifier
The gain resistor for the TIA (RTIA) and load resistor for the sensor (RLOAD) are external components.
Control Amplifier
The control amplifier maintains the sensor's reference electrode (RE) at the Bias Reference Voltage using
one analog continuous time block (CT BLOCK (PGA)) and one analog switched capacitor block (SC
BLOCK).
The voltage at RE is buffered via the CT BLOCK (PGA) and fed to the CB arm of the SC BLOCK. REFHI
drives the CA arm of the SC BLOCK. The opamp in the SC BLOCK servos its output to maintain its
inverting input at the same level as the non-inverting input, which is at AGND. Therefore, the node at
which CA and CB are connected is also maintained at AGND. Since the switch phases of CA and CB are
opposite, the voltages across CA and CB subtract.
Solving for PGA_OUT we have,
We also know,
Substituting Equation 3 for PGA_OUT in Equation 2 and solving for VRE we get,
TIA
The TIA maintains the working electrode, WE, at ground potential through the resistor RG, and provides a
voltage output proportional to the current through WE.
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GasSensorAFE
DC and AC Electrical Characteristics
Table 1.
5.0V PGA AC Electrical Characteristics
Parameter
Description
Min
Typ
Max
Units
IRE
Input leakage current
through the RE pin
–
–
1
nA
ICE
Minimum sink/ source
1
current through the CE pin
–
–
mA
VOS RW
Offset voltage between WE –
and RE pin
–
10
mV
VRW
RE-WE bias voltage error
–
2
% of FS
–
Conditions and Notes
Placement
The Gas Sensor AFE User Module uses three analog PSoC blocks, two continuous time (CT), and one
switched capacitor (SC). The Gas Sensor AFE also uses two output buffers and the band gap reference
generator.
Parameters and Resources
Bias Voltage
This parameter specifies the voltage, in millivolts, that the control amplifier maintains between the RE
and WE terminals.
TIA_Output
The TIAMP block output can be routed through an analog output bus. Setting the TIA_Output parameter to Disable, the default value, restricts the set of possible connections to the local network. If
TIA_Output is enabled on to the analog bus, make sure that no other user module drives this same
bus.
SCA_Output
The SCAMP block output can be routed through an analog output bus. Setting the SCA _Output
parameter to Disable, the default value, restricts the set of possible connections to the local network.
If SCA _Output is enabled on to the analog bus, make sure that no other user module drives this same
bus.
Application Programming Interface
The Application Programming Interface (API) routines are provided as part of the user module to allow the
designer to deal with the module at a higher level. This section specifies the interface to each function
together with related constants provided by the “include" files.
Note
In this, as in all user module APIs, the values of the A and X register may be altered by calling an API
function. It is the responsibility of the calling function to preserve the values of A and X prior to the call if
those values are required after the call. This “registers are volatile" policy was selected for efficiency
reasons and has been in force since version 1.0 of PSoC Designer. The C compiler automatically takes
care of this requirement. Assembly language programmers must ensure their code observes the policy,
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GasSensorAFE
too. Though some user module API function may leave A and X unchanged, there is no guarantee they
will do so in the future.
For Large Memory Model devices, it is also the caller's responsibility to preserve any value in the
CUR_PP, IDX_PP, MVR_PP, and MVW_PP registers. Even though some of these registers may not be
modified now, there is no guarantee that will remain the case in future releases.
GasSensorAFE_Start
Description:
Configures the Analog CT and SC blocks to function as a Gas Sensor AFE.
C Prototype:
void
GasSensorAFE_Start(BYTE bPowerSetting)
Assembly:
lcall
GasSensorAFE_Start
Parameters:
bPowerSetting: Gas AFE operating power.
Symbolic Name
Value
GasSensorAFE_OFF
0
GasSensorAFE_LOWPOWER
1
GasSensorAFE_MEDPOWER
2
GasSensorAFE_HIGHPOWER
3
Return Value:
None
GasSensorAFE_SetPower
Description:
Changes the power setting of the GasSensorAFE.
C Prototype:
void
GasSensorAFE_SetPower(BYTE bPowerSetting)
Assembly:
lcall
GasSensorAFE_SetPower
Parameters:
bPowerSetting: Gas AFE operating power.
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GasSensorAFE
Symbolic Name
Value
GasSensorAFE_OFF
0
GasSensorAFE_LOWPOWER
1
GasSensorAFE_MEDPOWER
2
GasSensorAFE_HIGHPOWER
3
Return Value:
None
GasSensorAFE_SetBias
Description:
Sets the bias voltage between the RE and WE pins.
C Prototype:
void
GasSensorAFE_SetBias (int iBias, int iExtRef)
Assembler:
lcall
GasSensorAFE_SetBias
Parameters:
iBias: Bias voltage in millivolts, 0–999.
iExtRef: The external reference voltage in millivolts, 0–2500, if external reference is enabled in the
global parameters, otherwise this parameter is ignored.
Return Value:
None
GasSensorAFE_Stop
Description:
Stops the Gas Sensor AFE by turning off the power to the Analog CT and SC blocks.
C Prototype:
void
GasSensorAFE_Stop
Assembler:
lcall
GasSensorAFE_Stop
Parameters:
None
Return Value:
None
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GasSensorAFE
Sample Firmware Source Code
The sample code in C is as follows:
//---------------------------------------------------------------------------// C main line
//---------------------------------------------------------------------------#include <m8c.h>
#include "PSoCAPI.h"
// part specific constants and macros
// PSoC API definitions for all User Modules
void main(void)
{
// Start the GasSensorAFE UM in the medium power mode
GasSensorAFE_Start(GasSensorAFE_MEDPOWER);
// Set the bias voltage between RE and WE to 300 mV and set the external reference
//(P2[6]) to 1000 mV.
// If reference is internal, iExtRef parameter is ignored.
GasSensorAFE_SetBias(300, 1000);
while (1);
}
The equivalent code in assembly is:
;----------------------------------------------------------------------------; Assembly main line
;----------------------------------------------------------------------------include "m8c.inc"
; part specific constants and macros
include "memory.inc"
; Constants & macros for SMM/LMM and Compiler
include "PSoCAPI.inc"
; PSoC API definitions for all User Modules
export _main
_main:
;Start the GasSensorAFE UM in the medium power modes
mov
A, GasSensorAFE_MEDPOWER
call GasSensorAFE_Start
;Set the bias voltage between RE and WE to 300(0x012Ch) mV and set the external
;reference (P2[6]) to 1000(0x03E8h) mV.
;If reference is internal, iExtRef parameter is ignored.
mov
A, 0x03
push A
mov
A, 0xE8
push A
mov
A, 0x01
push A
mov
A, 0x2C
push A
call GasSensorAFE_SetBias
.terminate:
jmp .terminate
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GasSensorAFE
Version History
Version
Originator
Description
1.0
KUK
Initial version
1.10
MYKZ
1. Added code generation script to read Ref Mux Global parameter and initialize the SC
block settings properly.
2. Moved UM to Legacy state as this is no longer recommended for new designs.
Note
PSoC Designer 5.1 introduces a Version History in all user module datasheets. This section documents high level descriptions of the differences between the current and previous user module versions.
Document Number: 001-81375 Rev. *A
Revised May 15, 2013
Page 8 of 8
Copyright © 2012-2013 Cypress Semiconductor Corporation. The information contained herein is subject to change without notice. Cypress Semiconductor Corporation assumes no responsibility
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assume any liability arising out of the application or use of any product or circuit described herein. Cypress does not authorize its products for use as critical components in life-support systems
where a malfunction or failure may reasonably be expected to result in significant injury to the user. The inclusion of Cypress' product in a life-support systems application implies that the manufacturer
assumes all risk of such use and in doing so indemnifies Cypress against all charges.
Use may be limited by and subject to the applicable Cypress software license agreement.