ATA5577C [DATASHEET]
9187H–RFID–07/14
20
5.12.3 Fixed-bit-length Protocol
In the fixed-bit-length protocol, the time between two gaps is nominally 24 field clocks for a 0 and 56 field clocks for a 1.
When there is no gap for more than 64 field clocks after a previous gap, the Atmel® ATA5577C exits the downlink mode. This
protocol is compatible with the Atmel T5557/ATA5567 transponder.
Figure 5-12. Fixed-bit-l en gth Prot oc ol
5.12.4 Long-leading-reference Protocol
To achieve better downlink performance, an enhanced Atmel ATA5577C reader places a reference pulse in front of the
opcode. This reference pulse is used as a timing reference for all following data, thus providing an auto-adjustment for
varying environmental conditions. The long-leading-reference protocol allows full compatibility and coexistence of both
Atmel T5557/ATA5567 and Atmel ATA5577C devices with both Atmel T5557/ATA5567 compatible readers and advanced
Atmel ATA5577C readers. However, only the Atmel ATA5577C devices can profit from the self calibration and the resultant
increase in write distance (see Table 5-1 on page 8 for option register settings).
In this mode, the reference pulse in front of the command is monitored. Depending on the pulse length, the remainder of the
command is either evaluated using the fixed-bit-length protocol, or is used as a measurement reference to evaluate the
following command bits. Otherwise, the following bits are considered as an invalid command.
a) For a reference-based command, the reference pulse (dRef) will have a length of 16 to 32 + 136 = 152 to 168 field clocks
(zero bit + timing bias = reference pulse). Hence, the expected length will lie between 152 and 168 field clocks. The
equivalent expected zero-bit length is then extracted and used as a reference for all following bits. The long-leading-
reference pulse in this case is used as a timing reference only, and does not contribute to the command data itself (see
Figure 5-13, part a on 21).
b) Should the first bit lie within the fixed-bit-length frame (for example, in normal mode: 0: 16 to 32 clocks; 1: 48 to 64 clocks),
the device will then automatically switch to the fixed-bit-length protocol (see Section 5.12.3 “Fixed-bit-length Protocol” on
page 20) and this first pulse will be evaluated as the first command bit. This allows compatibility with long-leading-reference
programmed Atmel ATA5577C devices interacting with Atmel T5557/ATA5567 readers, which do not send any reference
pulses (see Figure 5-13, part b on 21).
c) If an Atmel T5557/ATA5567 device interacts with an enhanced Atmel ATA5577C reader, the reference pulse (152 to 168
field clocks) is ignored by the Atmel T5557/ATA5567 and the following data bits will evaluated correctly. Therefore, an Atmel
T5557/ATA5567 device is compatible with an enhanced Atmel ATA5577C reader (see Figure 5-13, part b on 21).
d) Should the first bit correspond to neither (a) nor (b) then it will be rejected as an invalid command.
Table 5-10. Downlink Data Coding Scheme with Fixed-bit-length Protocol
Parameter Remark Symbol
Normal Downlink Fast Downlink
UnitMin. Typ. Max. Min. Typ. Max.
Start gap Sgap 815 50 815 50 Tc
Write gap Wgap 810 20 810 20 Tc
Write data
coding (gap
separation)
0 data d016 24 32 812 16 Tc
1 data d148 56 64 24 28 32 Tc
Note: All absolute times assume TC = 1 / fC = 8µs (fC = 125kHz)