ST AN4125 APPLICATION NOTE
AN4125
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Application note
How to manage simultaneous I²C and RF
data transfers with the M24LRxxE-R
Introduction
The M24LRxxE_R is an EEPROM device designed to be accessed via two different
interfaces: a wired I²C interface and a standard contactless ISO 15693 RFID interface.
Figure 1. Typical application of an M24LRxxE_R dual interface EEPROM
ST has published various supporting application notes explaining how the RF interface
works and the basic principles of passive RFID technology. These documents are available
from: www.st.com/dualeeprom.
The possibility of using two different interfaces to control the dual-interface EEPROM implies
two host controllers: a microcontroller with an I²C bus and an ISO 15693 RFID reader. Due
to their nature, these two host controllers are not synchronized, which means that both
controllers might try to access the M24LRxxE_R concurrently.
To manage this kind of situation, the M24LRxxE_R has a built-in circuitry able to handle
possible concurrent communications and powering activities from the RF and I²C sides.
This application note describes how the M24LRxxE_R arbitration circuitry operates. It
applies to the products listed in Tab le 1 .
Table 1. Applicable products
Type Part numbers
Memory products M24LR04E-R, M24LR16E-R, M24LR64E-R
June 2012 Doc ID 023297 Rev 1 1/15
www.st.com
Contents AN4125
Contents
1 RF - I²C arbitration mechanism description . . . . . . . . . . . . . . . . . . . . . . 5
1.1 Communications and power supply conditions . . . . . . . . . . . . . . . . . . . . . 5
1.2 Communication arbitration when the RF and I²C channels are
both active . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.1 I²C busy states . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.2 RF busy states . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.3 Arbitration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2 Recommendations when developing the
M24LRxxE_R application software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1 Issuing a command through the I²C channel . . . . . . . . . . . . . . . . . . . . . . . 9
2.1.1 I²C request while the RF channel is busy . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1.2 I²C requests and RF time slots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.1.3 An I²C request was interrupted . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2 Issuing a command through the RF channel . . . . . . . . . . . . . . . . . . . . . . 13
3 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2/15 Doc ID 023297 Rev 1
AN4125 List of tables
List of tables
Table 1. Applicable products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Table 2. Four possible combinations of power supply sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Table 3. Possible cases of communication arbitration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Table 4. M24LRxxE_R status according to command and V
Table 5. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
supply . . . . . . . . . . . . . . . . . . . . . . 13
CC
Doc ID 023297 Rev 1 3/15
List of figures AN4125
List of figures
Figure 1. Typical application of an M24LRxxE_R dual interface EEPROM . . . . . . . . . . . . . . . . . . . . . 1
Figure 2. I²C read command busy state. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 3. I²C write command busy state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 4. RF read command busy state. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 5. RF write command busy state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 6. RF Stay Quiet command busy state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 7. Example of an Inventory command where the M24LRxxE_R is decoded in
Slot 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 8. I²C polling when the RF channel is processing a command . . . . . . . . . . . . . . . . . . . . . . . . . 9
Figure 9. M24LRxxE_R state transition diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Figure 10. Optimal hardware schematic of an M24LRxxE_R application . . . . . . . . . . . . . . . . . . . . . . 12
4/15 Doc ID 023297 Rev 1
AN4125 RF - I²C arbitration mechanism description
1 RF - I²C arbitration mechanism description
The M24LRxxE_R arbitration circuitry is twofold. It contains:
● a power management unit that handles the power coming potentially from the RF or the
I²C side
● a communication arbitration unit that tackles potential concurrent communications from
the RF and the I²C sides
1.1 Communications and power supply conditions
The power supply management unit has been designed to allow for flexibility, especially
when both the RF power and the wired power line are active at the same time.
The basic principle is:
● When supplied only from the RF side:
– the M24LRxxE_R can be accessed only by the RF reader
● When supplied from both the V
– the M24LRxxE_R will serve the first decoded command (either RF or I²C) and will
not decode any command from the other interface (either I²C or RF) until the first
decoded command is complete.
Table 2. Four possible combinations of power supply sources
pin and the RF field:
CC
Possible
cases
Case 1
Case 2
Case 3 On
Case 4 On
1. VCC is “On” when the value is between VCCmin and VCCmax. Please refer to the M24LRxxE_R datasheet
for full details.
V
CC
0 V or not
connected
0 V or not
connected
(1)
(1)
RF field Actions
Off The M24LRxxE_R is reset.
On
On
Off
RF data transfers: yes
I²C data transfers: no
RF data transfers: yes
I²C data transfers: yes
(see Section 1.2: Communication
arbitration when the RF and I²C channels
are both active for details).
RF data transfers: no
I²C data transfers: yes
Doc ID 023297 Rev 1 5/15
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