If the host desires to send a byte to the QT it should behave as
follows:
3 Serial Communications
These devices can use either SPI or UART communications
modes; it cannot use both at the same time. The part defaults to
SPI mode unless it receives a byte over the UART interface. If a
UART byte is received at any time, the UART interface is
enabled and the SPI interface is totally disabled until after the
next device reset.
1. If DRDY is low, wait
2. If DRDY is high: send a command to QT
3. Wait 20µs (time S5 in Figure 3-3: DRDY is guaranteed to
go low before this 20µs expires)
4. Wait until DRDY is high (it may already be high again)
5. Send next command or a null byte 0x00 to QT
The host device always initiates communications sequences;
the QT is incapable of chattering data back to the host. This is
intentional for FMEA purposes so that the host always has total
control over the communications with the QT60xx6. In SPI
mode the device is a slave, so that even return data following a
command is controlled by the host. In UART mode, the device
will still only respond back to the host after a command, but the
responses are not controlled by the host.
It takes up to 1ms for DRDY to go high again after a command,
except for a few commands listed in Section 4:
0x01 (Setups load):
0x0E (Get eeprom CRC):
0x16 (Sleep):
<20ms
<20ms
<5ms
Other DRDY specs:
Min time DRDY is low:
1µs
A command from the host always ends in a response of some
kind from the QT. Some transmission types from the host or the
QT employ a CRC check byte to provide for robust
communications.
Min time DRDY is low after reset:
1ms
3.2 SPI Communications
SPI mode is selected by default after reset. There is no other
configuration required to make the device operate in SPI mode.
If a UART byte occurs before or even after SPI transmissions
have taken place, the device will switch to UART mode and
remain in that mode until the device is reset.
A DRDY line is provided that handshakes transmissions.
Generally this is needed by the host from the QT to ensure that
transmissions are not sent when the QT is busy or has not yet
processed a prior command. In UART mode this line is
bi-directional, and the QT can use it to suspend transmissions
back to the host if the host is busy.
Initiating or Resetting Communications: After a reset, or,
should communications be lost due to noise or out-of-sequence
reception, the host should send a 0x0f (return last command)
command repeatedly until the compliment of 0x0f, i.e. 0xf0, is
received back. Then, the host can resume normal run mode
communications from a clean start.
Figure 3-1 Basic SPI Connections
Host MCU
QT60xx6
Poll rate: The typical poll rate in normal ‘run’ operation should
be no faster than once per 10ms; even 50ms is more than fast
enough to extract status data using the 0x06 command (report
first key: see page 15) in most situations. Streaming commands
like the 0x0d command (dump setups: see page 15) or
multi-byte response commands like 0x07 or 0x08 can and
should pace at the maximum possible rate.
P_IN
P_OUT
SCK
MISO
MOSI
DRDY
SS
SCK
MISO
MOSI
Run Poll Sequence: In normal run mode the host should limit
traffic with a minimalist control structure (see also Section 4.23).
The host should just send a 0x06 command until something
requires a deeper state inspection. If there is more than one key
in detect, the host should use 0x07 to find which additional keys
are in detect. If there is an error, the host should ascertain the
error type based on commands 0x0b and 0x0c and take
appropriate action. Issuing a 0x07 command all the time is
wasteful of bandwidth, requires more host processor time, and
actually conveys less information (no error flags are sent via a
0x07 command).
Figure 3-2 Filtered SPI Connections
Host MCU
QT60xx6 Circuit
X drives
P_IN
P_OUT1
SCK
DRDY
Xn
Yn
Ra
Ra
Ra
Ra
(1 of 8
Ca
shown)
SS
Ca
Ca
SCK
MISO
MISO
MOSI
3.1 DRDY Pin
Ca
Y Lines
(1 of 6
DRDY is an open-drain output (in SPI mode) or bidirectional pin
(in UART mode) with an internal 20K ~ 50K pull-up resistor.
MOSI
Ra
1K
shown)
Ca
Serial communications pacing is controlled by this pin. In either
UART or SPI mode, the host is permitted to send data only
when DRDY is high. In UART mode, the device additionally will
hold up responses to the host if DRDY is being held low by the
host. After a byte is received DRDY will always go low even if
only for a few microseconds; during this period the host should
not send data. Therefore, after each byte transmission the host
should first check that DRDY is high again.
P_OUT2
RESET
1nF
Recommended Values of Ra & Ca
SPI Clock Rate
Ra
Ca
4MHz
680
33pF
270pF
470pF
1nF
400kHz
1,000
2,200
2,200
100kHz
50kHz
lQ
11
QT60486-AS R8.01/0105
QUANTUM [ QUANTUM RESEARCH GROUP ]
